I'm 54 and I started cycling in the fall of 2000. I rode a lot when I was a kid, up until age 14 or so (wish I still had my classic Raleigh with the generator light and a Brooks saddle). Before I started riding I was heavily involved in aerobics and other gym stuff.
When I started cycling, I lost the last 5 lbs. of the 10 I had been trying to lose. I was very skinny when I was doing aerobics, but I got sick of it and had gained a bit in my late forties.
I ride because I can be competitive, social, or just be relaxed while I am riding. I am competitive against myself only; I was the kid who got picked last in gym class. I have new friends because of cycling, some of them from this list! Best of all, I have a whole new lifestyle, based on being outside. Not only am I cycling, but I am hiking, walking, nordic skiing, and snow shoeing. Yes, I still belong to a gym because I go to yoga, spin class, and use the weights, but I have developed quite a home gym, with weights, balls, and my bike on the trainer for the winter. Maybe next year I might join a place just to do yoga.
I weigh the same as I did when I graduated high school.