I've met people who look 25 but are actually 45. A few of them, actually. It's on the extreme end of the bell curve but it happens.

As for more extreme cases, I could believe that a few people discovered that they weren't aging and hid it, but not more than a few people, especially in modern times. Switching identities isn't exactly easy and the probability of every one of them successfully carrying it out gets exponentially suppressed as the number of people increases.