As a guy in my late 30's, looking from the other side of the fence as it were, I personally don't think I could date anyone younger than their late 20's. As for older? I'm not sure - mid 40's perhaps?

I guess the personalities of the people involved has a large bearing on the way things develop - two people very close in their life outlook but far apart in age would make for a good match, just as two close in age but being in very different stages in their life would likely be disasterous.

In saying that, a guy 20 years older laying it on thick with sexts and mysterious rendezvous and things like that automatically puts me on edge. It definitely comes across as creepy to me. If they were serious about a relationship, then everything should be out in the open. Yes, it does leave you open for close scrutiny, but if those outside the realationship can see openness and honesty, then the suspicion should fade away.