It depends on where you go. The rule is if you look under 25, expect to be id'd but I went to London last year and wasn't id'd anywhere and I look about 17.
TBH i look so much forwards to it. My first steel city derby and the look on my brothers face when i told him i would take him to England to watch a game was all the money worth.
Only downside is that he might end up as a united fan instead of a top flight team with all the pains it gives :P
They've done studies, you know. 60% of the time it works, every time.