Further,

animals differ from one another in regard to the time
of life that is best adapted for sexual intercourse.

To begin with, in most animals the secretion of the seminal
fluid and its generative capacity are not phenomena simultaneously
manifested, but manifested successively. Thus, in all animals, the
earliest secretion of sperm is unfruitful, or if it be fruitful the
issue is comparatively poor and small. And this phenomenon is
especially observable in man, in viviparous quadrupeds, and in
birds; for in the case of man and the quadruped the offspring is
smaller, and in the case of the bird, the egg.

For animals that copulate, of one and the same species, the
age for maturity is in most species tolerably uniform, unless it
occurs prematurely by reason of abnormality, or is postponed by
physical injury.