I turn 42 this month.
Been riding for about 5 years now. Before that, not since I was in grade school.
Why? I trashed my knee running in college, and spent a lot of the next 10 years or more out of shape. Got my first bike after giving up my horses...found the freedom was the next closest thing.
Why do I keep it up? I love the combination of training goals & achievement, coupled with the joy, freedom and endorphin rush of working out, outside.
Weight loss? It goes up and down, depending on my eating. But, my legs and butt have really firmed up, and my body image has really improved since I've discovered my body can carry me up and down mountains all day on my bike, regardless of how it looks.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury