Breasts are for feeding babies.
They don't. Normally they grow during puberty, which happens before adulthood.
As to exactly "when" a girls breasts start to grow, that depends on heredity, diet and the onset of puberty. On "chubby" pre-teens they can start as early as age 8, but the average onset of puberty in girls is 10 1/2, with a range of 8 - 13.
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