Depends on how old someone looks. Some people just naturally look young for their age and casting them as younger doesn't pose a problem. It's more of a problem if an actor is in their mid 20's or 30's posing as a teenager. It then start to look a little ridiculous. Had started watching a recent series on netfilix where the character was supposed to be a teenager, but the actress was in her 30's and she came away sounded way more experienced at life than she should have been. The part of the teenager didn't come across as convincing at all.