你几岁？
Literally You how many years

Ni ji sui?
几 means how many
岁 means and is also a counter for years
Pronounced like “knee gee sway”

This is the basic way to ask a person their age. Similarly to a previous post, you can replace the question word, 几 “ji” how many with a number to answer the question.

你几岁？
我十八岁。
Ni ji sui?
Wo shiba sui.
How old are you?
I’m eighteen years old.