If you have a blonde toddler, the question keeps coming up at the playground: "Will it stay this color?" The honest answer is: probably not.
Hair color is governed by two pigments called eumelanin (brown/black) and pheomelanin (red/yellow). The genes that control them — MC1R, TYR, OCA2, and a few others — don't finish "switching on" until somewhere between age 2 and age 8. For most kids, hair gets gradually darker until puberty.
What predicts where it lands
The single best predictor is the parents' childhood hair color, not their adult hair color. If both parents were blonde as kids and went brunette in their 20s, your child will probably do the same. If one parent stayed light into adulthood, the odds of staying light double.
Sun exposure also matters more than people think — kids who spend a lot of time outside in early childhood often hold their light color a year or two longer.
When does it stop changing?
For most kids, by age 5 you can see roughly where things are heading. By age 10, the color is usually close to final. Puberty can push it one shade darker but rarely changes the basic family.