Cheap Trick, Canterbury Odeon 1978. I was 18 and had just arrived at University. It was the first gig I went to that I wasn't playing at
(I'd been playing out for about 2 years at that point). I think I saw The Cure (humped gear for that one), Ozzy, and The Pretenders all in the following couple of months.
Why did I wait so long? Well, the only way you could get to see a touring act when I was a kid was to go to Leeds. Nobody our age had a car, so unless you felt like either leaving before the headliner was half finished you'd be walking home 12-15 miles in the wee small hours. Some guys did it though.