Page 109 of Vampire the Masqurade mentions, “A 14th generation Kindred, who by miracle of avoided the flaw of thin blooded themselves…”

This establishes that a 14th generation may or may not be a thin-blood. Any person that a vampire creates will be a thin-blooded vampire. Almost all SPCs of the 14th generation will be thin-blooded. Players are exceptional and can choose if they are or if they are not.

A character of 14th generation may take any thin-blood merit as their blood potency is at the level of a thin blood.

Thin Blood Merits have the balance built into them (pg 183 VTM) and do not require an additional flaw to balance out. As with other merits and flaws, they may be purchased during play with experience points.