It is advisable to exercise as often as possible, without of course straining your body or driving yourself to injury.
Getting a trainer should be the first thing, as trainers are suited to developing a proper schedule for your particular needs.
You want to make sure that you doing the right exercises in the right way. Some people might end up with injuries is what Brad Reifler is saying, because they could not seek the guidance of a trainer. It is always good to start with a trainer so that even if later you choose not to exercise alone, chances of injuries are minimum.
You do not have to wait until someone tells you to start exercising. It is all about watching and listening to the body, to determine the best time.
Age is important in determining the kind of exercises you should do. The age of a person will help to determine the body endurance to exercising. Take an example of Zumba dance classes; there are different types of Zumba.
There is Zumba for kids and Zumba for adults. You can expect the Zumba for kids to have less complex moves than that for adults. Still there is Zumba Gold for people who would want less intense Zumba.
This can be suitable to people who are of old age. If you strain the body past some points it cannot endure, then it will snap resulting into some serious injuries.