Well too young can be a problem, same as being too old looking, women like men over 30 and looking under 40, they are more mature and financial well set usually. A girl a little bit older(over 25) would be a better target for you I am guessing, because she will feel more em-powered and likely give you a better time for your baht. But the young quiet ones can be right octopus after they get over the intial doubt about the guy- most are a little cynical about the bloke they'll think, dirt monger, drunk, butterfly....so talking simply and acting a little funny with them, whilst remaining gentlemanly will give you smiles about parting with your hard earned the next morning.
Biggest advise I would give to a young would be monger is- Don't drink heaps, limit yourself to say 6 scotch n drys(pre mixed in the bottle!) and drink only ginger ale after that, they also hate beer breath and smokers..... smoker BG's (not a lot of them do smoke) if they do smoke they usually have other nasty habits like yabba yabba.
Do wear a conny' mate what ever you do.