Efron: Viva Las Vegas—Except When You Turn 21!

Find out why you didn't catch the former High School Musical star jetting to Sin City when he turned 21

Zac Efron may only be 22 years old, but he's already mentoring young Hollywood.

At least when it comes to birthday celebrations in Las Vegas.

His 17 Again costar Sterling Knight tells us Mr. Efron has advised what not to do when he turns the big 2-1 next month...

"I've taken Zac's advice, and therefore I'm avoiding Vegas like the plague," says Knight, who stars opposite Demi Lovato on the Disney Channel series Sonny With a Chance. "He basically told me, 'You don't want to be that guy. You can be that guy at every other birthday, but at 21, have some class and be a polite and gracious host.'"

For those not up on their Efron trivia, he did practice what he preaches. Efron's 21st birthday celebration was a simple dinner at L.A.'s Pace restaurant with some family and friends.

In addition to party planning, Efron also offers career advice. "Zac says the hardest thing in the business is to not become jaded," according to Knight, who can also be seen in StarStruck, a television movie airing Sunday on the Disney Channel. "He says you have to become jaded in a sense, but you really need to focus on maintaining who you are, and enjoy the process while learning a lot."