I think the youngest I've met was 26, and there have been a handful of 27 - 30 year olds. I've seen so few younger men primarily because I don't get many inquiries from them.
Other than being at least 21, I don't turn down anyone
just due to age -- and that goes for the upper limit too. Old farts welcome!
However, regardless of your age, if you have a habit of typing like a dopey kid (text speak, incomplete sentences, creative punctuation), I'll assume you will behave similarly immaturely in person. And in my advanced age (*adjusts dentures, shakes cane*), I don't have the patience for that.