late bloomer

one who becomes mature at a later age than normal

A late bloomer is a person who does not discover his or her talents and abilities until later than normally expected. In certain cases, the individual may be as old as 60, and retirement may lead to this discovery. The late bloomer is something of a contrast to a child prodigy and as such they often occur in almost opposite fields.
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a person who is slow to mature or succeed Kelly is not doing well in math, but she may be a late bloomer.