bloomer
A person who develops or shows their abilities over time.
A bloomer is a person or plant that blooms, meaning they flower or develop. An early bloomer is a flower that opens in spring before most others, or a person who develops talents or abilities ahead of schedule. A late bloomer is the opposite: someone who takes longer to hit their stride. Some kids read fluently at age five; others don't click with reading until eight or nine but then devour books. Both can end up as great readers. The term reminds us that people grow at different paces.
Bloomers (with an “s”) are loose-fitting pants gathered at the knee, worn by women and girls starting in the 1850s.
In British English, a bloomer can also mean a foolish mistake: “Forgetting my permission slip was a real bloomer.” It's a gentle, slightly humorous way to acknowledge a mistake.