Free starbucks drink on birthday?

You have to be 18 years old to purchase alcohol. They say you have to be 21 years old but you have to be 18 and if you look younger then 21 then you are asked for ID. I don't really understand the system but anyways that's how it is. I think that maybe starbucks got this wrong because you really have to be 18, either that or the intention was a free non-alcoholic coffee drink which makes more sense to be honest. I'm not really one hundread percent sure if i'm right or not but i hope this helped.