"Emeka is now a mature student, but it took him years to mature."

The word "mature" is both an adjective and a verb. When it comes before a noun as in "mature student", it is an adjective used to describe a fully grown individual (in this case a fully grown student). When it is used as a verb, it means to fully become developed over time. 

Here are cases where "matured" can be used:

As a verb (past tense):

• "Over the years, Emeka has matured into a responsible adult."

In this sentence, "matured" is the past tense of the verb "mature."

As an adjective:

• "The wine has been carefully aged and matured to perfection."

In this case, "matured" is used to describe the wine that has undergone a process of ageing and development over time.