No idea what ipso-facto means but yeah, I remember thinking 30-year old were old when I was 20.
My point is that an 18 year old Thai girl does not find any male over say 25 attractive.
If the OP is 49, this is perverted that he wants to shag an 18 year old. Sadly, many perverts come to Thailand for exactly this reason.
Do you want to be in one more old farang's gang?