Why was I asked for my ID for a mocktail

A few weeks back I went to a bar/restaurant place with my parents after a busy day out. I don’t drink casually so just ordered a mocktail because I love them. The bartender asked for my id which confused me because it’s a non alcoholic drink. Im of age but just look really young. I didn’t have it on me so just ordered a juice instead. I understand that some bars refuse service to people they can’t id no matter what drink but he let me order juice so im just confused