Well you KNOW wrong <img src="/forums/images/graemlins/wink.gif" alt="" /> it's typically store policy for places like wal-mart to NOT sell handgun ammo to people under 21, but it is certainly not the law.

http://www.lcav.org/states/Texas.asp#MinimumAgetoPurchasePossess
this is the best reference I could find.

"There is no minimum age to possess firearms under Texas law, although federal law generally prohibits the possession of a handgun or handgun ammunition by anyone under the age of 18. 18 U.S.C. º 922(x)(2), (3) and (5).

Federal law prohibits firearms dealers from selling or delivering a shotgun or rifle, or ammunition for a shotgun or rifle, to any person the dealer knows or has reasonable cause to believe is under the age of 18. 18 U.S.C. º 922(b)(1), (c)(1). Dealers are prohibited from selling or delivering other firearms (e.g., handguns) or ammunition for those firearms to any person the dealer knows or has reasonable cause to believe is under the age of 21. Id."