zreti

1. (of fruit) to ripen
   Paradajz polako zri na suncu. — The tomatoes are slowly ripening in the sun.
   Grožđe zri krajem leta. — Grapes ripen at the end of summer.
2. (of a person, an idea) to mature; to grow up
   Ta ideja je dugo zrela u meni. — That idea had been maturing in my mind for a long time.
   Deca brže zru u teškim vremenima. — Children grow up faster in hard times.

In everyday speech sazrevati is more usual; the perfective is sazreti.