adulthood

Adulthood is the state of being fully grown. Reach adulthood, early adulthood, into adulthood.

The legal line is drawn at eighteen in most countries and the biological one is not drawn at all — the brain's prefrontal cortex goes on developing into the mid-twenties, which is why insurers and criminologists both treat young adults as a separate group. Note the -hood ending, meaning a state, as in childhood.