I've noticed a pimple sized bump on the top of my head. It's painless and hasn't grown in size. I believe it's been there for years. Should I worry?

Probably no worries. Since the pimple sized area has been present and unchanging for years, and isn't bothering you it is very unlikely to be bad. It is probably a mole or a small cyst. If it changes size I'd have it checked.