What are you hoping to meet a nice 18-20 year-old girl for? If it's just for sex, then you're a sex tourist just like many others. If that concerns you don't do it!
If you're hoping to find a "girlfriend", then don't look in sex-tourist places like Hua Hin's bar areas or online (most dating sites are really for prostitution) but unless you speak Thai and are young and attractive, you'll find it difficult.
If you are successful in finding a much younger "girlfriend", either from a bar or anywhere else, you'll just have to put up with comments, and after a few years you won't even notice them.