sweetheart vs adult female

sweetheart and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweetheart	Yes	Yes	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of sweetheart; that is, adult female is a word with a broader meaning than sweetheart:

Other hypernyms of sweetheart include woman.

sweetheart (noun)	adult female (noun)
a person loved by another person	an adult female person (as opposed to a man)
a very attractive or seductive looking woman
any well-liked individual
a beloved person; used as terms of endearment

sweetheart (adjective)	adult female (adjective)
privileged treatment of a favored person or corporation (sometimes unethically)