How old is to old for a guy to have relations with a 18 to 21 year old girl?

We need to quit obsessing so much on the lives of others. If there's a book we worship more than a bible, it would be a calendar. It would bug me seeing a 60 year old with a 20 year old, but if they're happy so be it!

As long as they are both consenting adults, it is no one else’s business.

The typical rule is "half the age +7": for an 18 year old anything above 22 is creepy; for a 21 year old anything above 28 is creepy. "Wrong" is a different kettle of fish.

I’d say about 40. Except for me, then the answer is 120.

Older than 26. If my daughter, older than 23.

Frankly I would say 30. Honestly though, I am about to be 34 at the end of this month and am tempted to date someone in their early to mid 20s. Mainly cause I am single and cannot find a single woman my age who doesn't have kids.

Anything over 22.

depends on the people involved, really

a large age difference does let in opportunities for manipulation via money/property/employability leverage - an older person might know more manipulative techniques.

i like a more level playing field, being somewhat cynical about human relationships.