I see others answered just as I was opening the window to answer. Some say it is state by state, I was under the impression this was a federal law. Anyone know for sure (not guessing, know)?
I couldn't find any age-related prohibitions against selling ammunition in Texas PC §46. But I did find the following in 18 USC Sec. 922:
"(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver -
(1) any firearm or ammunition to any individual who the licensee knows or has reasonable cause to believe is less than eighteen years of age, and, if the firearm, or ammunition is other than a shotgun or rifle, or ammunition for a shotgun or rifle, to any individual who the licensee knows or has reasonable cause to believe is less than twenty-one years of age;"
So it appears to be a Federal thing (at least in Texas).