A newborn is helpless and depends on its parents for a long time: children usually take their first steps at 9–18 months, and puberty takes place at about 10–16 years. Growth continues until 18–20 years of age. Average life expectancy in most countries is 70–85 years, and many people live to 90–100. The longest documented lifespan is 122 years, and scientists estimate the theoretical limit of human lifespan at roughly 120–150 years.