This is the 13518th most frequent Polish word.
pełnoletnia
"Pełnoletnia" means "adult" or "of legal age" (feminine form).
In this sentence, 'pełnoletnia' is an adjective describing Maria as someone who has reached the legal age of adulthood.
Maria jest pełnoletnia i może legalnie głosować.
Maria is of legal age and can vote legally.
Here, 'pełnoletnia' is used as a predicate adjective referring to a person reaching legal adulthood.
Kiedy osoba staje się pełnoletnia, może podejmować samodzielne decyzje.
When someone becomes of legal age, they can make independent decisions.
In this usage, 'pełnoletnia' is an adjective qualifying 'osoba' to indicate the need for someone who is legally an adult.