Generally in development, maturation stops once adult size is reached (at least in most animals - a few grow throughout life). This tends to coincide with sexual maturity i.e. when the animal becomes reproductively active.

But the process can be different for different parts of the body. For example, humans reach maximum number of neurons just before they are born - from then on it's downhill all the way. But brain maturation continues until about age 20 with myelination of axons.