As long as you don't have any severe medical conditions, you can start at any age. Just get a bike that fits you well, pump up the tires and ride. If you like it, you can always upgrade. It's great to get with a group of people who are at our level and set goals regarding time and distances per week or month.
Cycling is fun and there are different types of cycling. I enjoy road rides with my friends. It pushes me to improve my fitness and overall strength. Three or four hours on the bike might not be everyone's idea of fun. Mountain biking is also fun and carries with it a totally different list of challenges.
Whatever you choose to do, prepare to have a site butt got a few days or longer when you start out. Soreness is to be expect. If you get any numbness, you need a new saddle or your bike fit is off.
PS - I know lots of 55 year olds who routinely crush younger riders on group rides.