A lot depends on your lifestyle and what you hope to achieve by training, so lets concentrate on a normal level of fitness, the level of fitness is defined by the ability of your lungs to extract oxygen from the air, get it into the blood, and the ability of the heart to pump that oxygenated blood to where it's needed during physical activity.
Your body style really has less to do with it, don't be misled into thinking that one who looks good on the outside is going to be fit.
A healthy body, as opposed to a fit body, is not something that is achieved so much by training but more by what you put into it, ie food, and unless that is right a lot of training is really going to be wasted time.
If you eat right, lead an active life, ie take part in some sport, walk some,or have a job that involves physical activity, then there is really no reason why you should need to train at all. Oh and by the way age is NO barrier to achieving normal physical fitness.
Of course there are other reasons people train, build muscles for example, training their body to handle a particular sport, weight loss is another you will hear but imo that is best done in the kitchen, I am not saying you should not train and if you know you are "unfit " then of course you should do so. Borders cannot be applied generally, it depends on you.