You can be 18 to purchase scratch offs and lotto tickets but you gotta to be 21 to turn them in if you won?

Somebody explain this to me. I'm 19 I buy lottos for my cousin and for other older family members and the guy in the corner store is telling me I can't purchase any because I'm not 21 but on the back of everything dealing with lotto and scratch offs they say you have to 18 years of age right so now my question is when a person when's something off of any of those things, do you have to be 21 to collect???