TORONTO - In the coming months, Canadian pop sensation Justin Bieber will reach a milestone that could change the course of his career. He's turning 17 in March.

Sure, the next months will also find the mop-topped crooner competing for his first two Grammy Awards and performing for his swooning fans in Europe, the Middle East, Australia and Asia.

But as Bieber ages, the most pressing question becomes: will his adoring fans stick with him as he continues to progress through puberty and beyond?

The last year has been an amazing success for Bieber, by any measure.

His "My World 2.0" topped charts around the world en route to double platinum sales in Canada and the U.S. He flexed his marketing muscle with an endless series of merchandising agreements, lending his fresh-faced stare to trading cards, dolls (or "action figures") and most bizarrely, a line of nail polish.

He published a memoir, guest-starred on "CSI," and kept plenty of fans up past their bed times with an appearance on "Saturday Night Live."

And perhaps no other celebrity has been as savvy with social media. Bieber has amassed an amazing online following who titter and Twitter so much about the singer that he wound up among the most-searched figures on virtually every major online platform, including Google and Facebook.

Bieber's big year culminated in nominations for best new artist and best pop vocal album at the 53rd Grammy Awards, scheduled for Feb. 13 at the Staples Center in Los Angeles.

Such accolades could be the beginning of Bieber's shift toward a career as a credible adult pop artist, or they could simply be the cap on a particularly phenomenal teenybop sensation.

At this point, it's tough to predict which way he will go.

"The odds are against him -- 99 per cent of teen idols don't make it," said Brad Schwartz, vice-president of Much MTV Group, in a recent telephone interview.

"Think back to your own youth. People do not grow up with the music they were listening to when they were 13. So we'll see how he handles that challenge. But I think he can do it."

Indeed, there's a long list of teen sensations who slid from view once they were old enough to vote. Others became public trainwrecks, relegated to reality-TV curiosity status.

The list of one-time teen sensations who failed to find success as adult artists is long and well-documented, with nostalgic names including Leif Garrett, Shaun Cassidy and Tiffany.

Even some of the relative success stories are marred by struggle and personal disappointment. Britney Spears, for instance, has managed to cling to a recording career as she approaches her 30s but has been in the news over the past decade more often for shaved heads, shotgun marriages and unfortunate limousine snapshots than for hit singles.

Sometimes these young artists succumb to drug or alcohol dependency, the pressures of public scrutiny, or simply to the fickle tastes of a youth-obsessed culture.

The Jonas Brothers, for instance, have arguably seen their stars dim slightly, particularly since Bieber's ascension. Nick Jonas's offshoot record, "Who I Am" -- released earlier this year, when Jonas was 17 years old -- barely made a ripple.

Those teen fans who now shriek at the mention of Bieber's name, shut down malls, and spend hours upon hours on their computers trying to catch his attention, will eventually grow up and, in many cases, they'll outgrow their crushes on Bieber.

However, there have been models of success for the singer to follow.

Coincidentally, two artists who best handled the transition from teen sensations to grown-up artists have links to the 16-year-old from Stratford, Ont.

Justin Timberlake appeared on "The New Mickey Mouse Club" (which also served as a showcase for Spears, Christina Aguilera and Oscar-nominated Canadian actor Ryan Gosling) as a child before joining 'N Sync, a bubblegum pop outfit that rivalled the Backstreet Boys for the adulation of millions of screaming pre-teens in the late '90s.

'N Sync released its self-titled debut in 1998, when Timberlake was 17. After releasing a fourth album, "Celebrity," in 2001, the group disbanded but Timberlake hardly missed a beat.

The next year, he released his solo debut, "Justified." The album was a major hit, and its innovative, hip-hop inflected production and Timberlake's elastic vocal performance won surprising critical kudos. His career as an adult artist was born, although the remaining members of 'N Sync haven't been so lucky.

When Bieber first caught notice for his YouTube videos, Timberlake was one of the first who tried to sign him, but he was beat out by Usher, another teen idol turned music mogul.

Usher released his debut when he was just 15. With his baby-faced good looks and Michael Jackson-influenced dance moves, he was a natural hit with the pre-teen set.

That he was able to translate those features -- which seemed inextricably linked to his youth -- into a career as one of the most popular R&B singers of his generation could serve as an inspiration for Bieber.

Yes, Bieber's singing voice will deepen, and the process has apparently already begun (Bieber said backstage at this summer's MuchMusic Video Awards that he was working with a vocal coach as those changes progressed). With time, who knows, maybe he'll even choose to lose his trademark shag.

But many music insiders believe it's a transition he can make successfully.

"Like I said, the odds are against him, but he's certainly looking like the real deal," Schwartz said. "It's going to come down to his talent, it's going to come down to what his second record sounds like, it's going to come down to how his career is managed.

"It's going to come down to a lot of things."

One thing working in Bieber's favour seems to be a slightly more favourable critical reaction to his work than his teen-pop forebears have received.

Where other bubblegum pop stars' music has been dismissed as fluff, Bieber has quietly gained positive press.

Though cynics would argue that his Grammy nominations might be a desperate bid for younger viewers from the greying music institution, the fact remains that other recent teen-pop stars didn't get Grammy recognition as quickly, or at all.

His "My World 2.0" received a generally favourable 68 score on review aggregator Metacritic, with Rolling Stone calling the collection a "seriously good pop record" and Entertainment Weekly remarking that "there's real talent, it seems, under all that hair."

And New York Times pop critic Jon Caramanica even placed "My World 2.0" on his list of the 10 best albums of 2010, praising Bieber as a "stealth ambassador for 1990s R&B" whose songs "merited close attention."

Bieber plays multiple instruments and his list of willing collaborators includes plenty of older, respected artists, including Kanye West, Raekwon, Ludacris and Rascal Flatts.

So, Bieber would appear to be on the fast track to real artistic credibility.

"The fact is, the industry fell in love with Justin Bieber, not just teen girls," said Simon Dumenco, a media columnist for Advertising Age magazine. "Even when the record came out, it wasn't dismissed. It was never dismissed as treacly garbage.

"And if you look at the collaborations that he engaged in, the music industry took him very seriously.... There's a ton of people who wanted to work with him."

Of course, many young artists who have been unable to extend their careers into adulthood have faltered due to the various temptations of fame, chemical or otherwise.

Chas Newkey-Burden, the British author of "Justin Bieber: The Unauthorized Biography," says that Bieber's mother, Pattie Mallette, is a steadying fixture by his side.

But he hasn't shown a proclivity for bad behaviour anyway, Newkey-Burden said.

"He was never really a big party animal," he said in a recent telephone interview from London.

"He's had such a wholesome upbringing. You know, the most scandalous story of him pre-fame that I found was that when he took his first date out, he spilled spaghetti and meatballs over himself.

"That's not quite the same as Keith Richards snorting up his grandfather's ashes because he thought it was cocaine."

Added MuchMusic's Schwartz: "At the MMVAs, he came with it -- he really did. He put time into the performance, he put time into his rehearsals, he wanted to kill it and he did. Working with him -- as far as MuchMusic is concerned -- he's professional, he's got great people around him, and that is key to long-term success."

And some have noticed that Bieber's transition has already begun.

Newkey-Burden notes that he's already straddling the line between a squeaky-clean teen-pop image and something a little more adult.

"He is at once the angelic, boy-band choirboy singer, singing on a stool, crooning about puppy love -- he's got that side to his image visually -- but he's also gone for this bad-boy thing," he said.

"He's gone down this Usher route, and they're trying to merge him into this sort of hybrid of an angelic choirboy and a street homey."

Bieber recently took time out of his tour schedule to appear in CBC-TV's holiday special, "Season of Song: Canadian Tenors and Friends."

The Tenors came away impressed and they also noticed a change in Bieber.

"The song that he sang on our special specifically is kind of that transition song, where it's a bit deeper than what we've heard in the past, it's almost an homage to Michael Jackson's 'Man in the Mirror,"' said the Tenors' Clifton Murray in a recent interview.

"The sentiment is: 'I have to make a change, and I pray for the world to be a better place.' ... You haven't heard stuff like that from him before.

"I think that's the start of him being conscious of, 'OK, (I'm) not going to be 16 forever. What can I do to make sure that I'm a relevant artist for years to come?'

"I think he's on the right path."