Asked for my ID buying 1 bottle of wine

Bought a bottle of wine to take to Scott and Amy's and the woman at the checkout in Raley's asked for my ID. Apparently they have to do that if you look under 30. I think this is ultra cautious legal guidelines to make sure nobody under 21 (?) gets booze. Blanket rule is that if you look under 30 you need to show ID. I told her I was closer to 40 and she said she only wanted to see my ID really (English accent thing maybe).