Why do people get adult acne?

I'm 15 (16 in April!) and I have fairly clear skin. I've never gotten like those mive breakouts a lot of teenagers get but occasionally I'll get like a random zit or two but they go away within the next few days. But anyway, I saw this Proactiv commercial about Mandy Moore and she said she was the same way - clear skin when she was a teenager, but when she got into her 20s she got adult acne. How does this happen? Is it because of stress/etc.?