Usage patterns
〈数〉歳になるto turn ... years old
He will turn 20 next month.
何歳ですかhow old are you?
Excuse me, but how old are you?
満〈数〉歳full ... years of age (as a legal/official age)
You have the right to vote from the age of 18.
〈数〉歳の〈人〉a person who is ... years old
A 5-year-old child is playing in the park.