Any age is legal. I am sure that someone might want to stop you if you were seemingly too young, but there really isn't an age limit.
Laws vary by state, but most states allow minors to buy condoms once they are in high school. Ideally, there is no age limit. It totally depends on you. If you want a safe sex and want to make smooth relationship with your girlfriend, it's good to buy condoms whenever you indulge into sex.
http://funadvice.com/r/15brqvmc7glCan the cashier lecture us about buying condoms?