Hair all over the body has a growing phase ( called 'anagen'), a resting phase (called telogen) at the end of which the hair falls off.
The rate of hair growth varies in different parts of the body. It is fastest on the scalp and slowest at the eyebrows. Since hair grows so fast on the scalp, the growth is noticeable.
On the legs and underarms the growth rate is very slow. By the time a hair completes its growth phase and reaches its telogen phase, it only manages to grow a few cms in the legs and a few inches in the underarms. Then it falls off to be replaced by a new hair.
Thus it appears that the hair at these two regions (as also in other places like the eyebrows) grow only to a certain length and not beyond that.
I hope this helps.
Possibly the hair follicles have degenerated with age. This is not very uncommon.
But other causes for loss of body hair are hypothyroidism, genetic factors, acute illnesses, certain medicines and a condition called alopecia areata which is an auto-immune disease.
If the loss of your body hair is quite extreme, you need to consult your doctor for a thorugh evaluation.