I think you should have called her out on it and say how inappropriate it is for you to mention her ex-bf dink size. That's like saying to her after you guys have sex that your ex-gf was tighter than...
If I were you, I'd be only interested in getting physical. Nothing else. 18 year olds and mid to late 20s have completely different mind sets and maturity. Don't expect anything long term to come out...