fetal age vs age

fetal age and age both are nouns.

Word	Noun	Adjective	Verb	Adverb
fetal age	Yes	No	No	No
age	Yes	No	Yes	No

As nouns, age is a hypernym of fetal age; that is, age is a word with a broader meaning than fetal age:

fetal age (noun)	age (noun)
the age of an embryo counting from the time of fertilization	how long something has existed
	a time of life (usually defined in years) at which some particular qualification or power arises
	a late time of life
	an era of history having some distinctive feature
	a prolonged period of time