Bonjour
How old are you ?
Polite you ► Quel âge avez-vous ?
Casual you ► Quel âge as-tu ?
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"vous" (formal "you) is used to someone you don't know, elderly person, boss...
"tu" (informal you, 2nd person sing) is used to friends, family, children....
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hope this helps ☺☺☺