late bloomer

Short definition from Wiktionary

Noun

1. 1
   A person who reaches puberty or who matures comparatively later than is typical.
2. 2
   A person who enters a romantic relationship or engages in sexual intercourse significantly later in life.
3. 3
   A person who achieves proficiency or success in a field significantly later in life.
4. 4
   A flowering plant that blooms later in a growing season than other flowering plants.