Selling liquor to minor is not an criminal offense.... for an asian guy like you and me it's hard to guess the age of the customer.. so it's quite understandable that you didn't ask for the ID before selling liquor to that chump. You paid fine and that's done.. it's one time happening.. don't worry about it. This is something like you getting traffic tickets for speeding.. ha ha...

You don't have to mention in the PR application form about this incident at all.. be assured !!

(P.S. I am not a lawyer)