how to tell your parents you want to date an older guy?

If age with your parents is really an issue and you think they will not be so approving, I would suggest just being upfront with them because in the end they can't tell you who to date and who not to date. Even bring him around to meeting them may loosen them up and they can see what a great person he is for you. Aslong as it's an equal relationship and it's positive, surely over time they will be more accepting.

Approach them sounding responsible; not as though you're worried, as though you've simply thought it all through. Say, 'Recently I've gotten really close with [whatever his name is] and we thought we might try dating for a bit' (make it sound as though you're not completely committed already, that will probably upset them) 'and I know since you care about me so much you might want to meet him, so I'll bring him round [whenever]. He's a twenty-year-old [student at uni studying this and that / whatever he is].' Drop it in casually, as if it's not something to worry about, just a fact about him.