Long term benefits and risks of bicycling
Long term benefits ... Hmmm ... does 55 years count as long term? If so, I think I and a few others here qualify ...

Key benefits


Cycling is a life-long form of exercise. One of the group I meet for coffee is 82, another is 80. A bunch are in their sixties and seventies, and another bunch are in their fifties. About a quarter are women. Another bunch I ride with has people from late teens through to sixties. Half are women.
Cycling can be done in a thousand ways. I've seen young professional Italian women cycling to work in high heels, Italian grandma's cycling to meet for coffee, and their menfolk kitted out in lycra for their daily "roll". You can race, tour, commute, shop, goof off, cycle around the world. It can be your only form of transport. You get places faster than walking or running, and see almost as much.
Cycling is low impact (unless you crash). If you choose to cycle with intensity, it is has far lower impact on your joints than activities such as running, jogging, the latest Gym fad, and even swimming (which can be damaging to elbows and shoulders).
Cardiovascular, aerobic capacity, strength, metabolic level. Most of the health issues of today's world are due to too little of these. You are going to be healthier, and healthier for longer, with a cycling life style.
Endorphins (feel good hormones) are released by cycling. You cannot stay grumpy or depressed with a cycling life style.
Office productivity. If you're a sedentary office worker, the higher metabolic rate and higher blood flow will give you more energy at work and higher productivity.
Weight and cholesterol control. The higher metabolic rate produced by regular physical activity enables your body to burn it's way through lots of fats and foods.
Social. Meet a significant other. Whatever.
Bedroom performance. For either sex, bedroom performance is better when you're fitter.
Being quicker than cars through inner city traffic.
Being able to bypass traffic jams.
Idealists may think they'll save money.


Key risks


Crashing, or being crashed. On a bike you are more vulnerable. The solution is to simply take 100% responsibility. See Best ways to avoid getting hit by cars?
There have been medical reports of lower sperm counts among (male) cyclists. This one has always been exaggerated. Your better general health is a far better indicator of reproductive success.
Owning too many bikes, each of which costs more than your car.
Er ... hard to find too many more. Too much vitamin A? Being annoyingly happy with life? Annoying your friends by knowing what a kilometer is?