I'm 44 and will have been biking for 2 years this march.
I keep with it because it it is so much fun. If weightloss were the only motivation, I'd have ditched it in the first month when I gained 9 pounds or so (without a change in eating habits). That first nine was joined by a second nine in the next couple of months, but I could tell I was getting smaller. I'm sure that quite a bit of that must have been muscle gain. Those pounds did eventually come off plus another 20 in the first year. Another 10-15 pounds came off this year, but the holidays were brutal since the weather was bad enough to interfere with riding (and the family situation has taken some depressing turns). I'm actually at the point where I don't care if I lose another pound (even though I need to lose at least 60). I ride because I love it.
I also ride with a group of people that I really like, which is helpful on those cold, dampish days when I'm balking at donning all those layers necessary to brave the weather