I'm 26, been once, when I was 10 or 11, primary school leaving trip, was a whistle stop tour of all the main sights at the time, downing street, Buckingham Palace, the mall, Horseguards, st pauls, big ben, think we went on a boat on the Thames, rode the tube, got train to Paddington.
I didn't enjoy it that much, and wouldn't be fussed if I never went back really.
But, I do like history and science and think I should visit some of the museums and exhibitions etc that unfortunately are only in London. I have a lot of friends/acquaintances who now live/work in London, and I have to say, on the whole, they come home much worse off for it. I do feel some peer pressure to go though, it's not very far away, and it's good to broaden your horizons. I also think I might appreciate it, or think differently about it at this age.
Like someone above said, after 2 days in a city, as much fun as is can be, I just can't wait to leave and have an overwhelming urge to just walk/run/cycle/drive as far as I can as fast as I can to get away from all the people/grime/traffic/rudeness.