If a girl is active and exercises a lot, can it delay her period?

CelineBV
My aunt's youngest daughter (cousin) is turning 15 on Dec 30 and still hasn't gotten her period yet, not at all. Plus she's underweight but gets hungry a lot. She's a bit worried.

Is it because of exercising a lot?
If a girl is active and exercises a lot, can it delay her period?
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