There's a comprehensive discussion of this topic in the legal forum:
"Texas Laws Regarding Underage Providers"
This is what I wrote in conclusion:
So what should agencies and hobbyists do to avoid criminal liability for dealings with underage providers? I always suggest the "Klong Rule." (A "klong" is a Yiddish word for the sick feeling a person gets in the pit of their stomach when they realize something is wrong, like when you pull your wallet from your pocket to pay a provider for the great time you've had and you realize you forgot to go to the ATM). The Klong Rule, applied here is:
If she looks as if she COULD BE 17 years of age or younger, then assume she IS 17 years of age or younger, no matter what she says or what's shown on a document [or a card, such as an ID] she produces.