Greg Lemond in some interview somewhere said what he didn't realize is how much of a toll the sun took on his skin (his face in particular) and that's one thing he'd change if he could.
Cycling for professional reasons is probably not that healthy - just like being any other professional athlete.
But taken into moderation, cycling reduces blood pressure, resting heart rate, flushes out your system, develops muscles without much impact problems, helps stabilize your kneecap (inner quad gets developed, unlike in running), makes for flexible quads, develops your glutes, and generally conditions your body. Cycling at a recreational level is a sport for all ages, although it seems like after 55-60 yrs old the number of racers drops sharply.
At the same time, too much cycling (either at once or a huge step up in mileage) can result in excessive fatigue, soreness, and numbness (hands, crotch, even feet). Like any other aerobic activity, your body needs to acclimate to different levels of activity.
If you go into it with professional aspirations, be realistic. Did you beat the club TT record the first time you did it while using sneakers and a borrowed bike? Or get disqualified from a win in a time trial or hill climb because everyone thought you cheated since you won by so much? These are signs that you should think about turning pro. If you aren't completely dominating everyone up to a Cat 2 level from the start then those are signs to think of cycling as a hobby. Although others may dispute this, I believe there is talent necessary at a genetic level for you to be competitive professionally. All the dope in the world won't help you overcome lack of talent. Lack of talent isn't a good or bad thing, it's just a fact of genetics. Chris Carmichael doesn't say "Oh Lance is one of the worst riders we've seen but he's worked really, really hard to get where he is". He says "Lance is an extremely talented racer, generates record numbers in his lactate/wattage/whatever, and then he trains really hard to maximize his talents."