Agree with others - no set age obviously! Was born with few black sparsey hairs, disappeared was bald til 1, then was white blonde until 4-ish, sandy, stayed varying shades of blonde/brown until about 12 when it went quite dark one winter and never turned back. Still very light brown though on the 'brown spectrum'. Also, my hair still to this day gets lighter in summer, quite blonde if I'm in the sun for most of the time.
DH has jet black hair.
DS1 born with black hair, has been blonde til now (nearly 9) still what I'd call 'dishwater blonde'.
DD1 born bald, white hair until age of 4, then it went mousy and it's about same colour as mine now.
DD2 born black hair, bald til 2, came in mousy, has darkened to darker than me but still not black.
DS2 born bald, is yellow blonde at the moment, he's 2.5
DS3 born head full of black hair (first one!) it's nearly all disappeared now but it's still dark.