Bloomer (noun)

A person or thing that is slow to develop or improve.

Origin:

From bloom + -er, from the full blooming of flowers.

Examples:

1. He was a late bloomer, only starting to excel in his field in his thirties.
2. The company was a bloomer, taking several years to become profitable.
3. The student was a late bloomer and only began to excel in her studies in high school.
4. The athlete was a bloomer and only began to win competitions in his late twenties.
5. The child was a bloomer and only began to talk at the age of three.