How old do you have to be to buy Airsoft BBs?

I just bought a aifsoft gun and it only came with 500 BBs. my 25 year old cousin was with me when i bought it because i know the rule where u have to be 18 to buy the actual airsoft gun. I was wondering if i could go out and buy airsoft BBs on my own when i'm only 16 or is there a limit?