nǐ jǐ suì？ 你几岁？--used to ask young people or kids. nǐ duō dà ？ 你多大？ --for common conversation nín duō dà niánjì ？ 您多大年纪？ ---to ask old people with respect P.S. you can use or omit "了le" at the at of each sentence which is just an constructional partical word.

if you asked the elder people, you may say 您老高寿？ but if you are talking to the kids , just saying:你多大了？ Sometimes we may make jokes with friends or persons we are familiar, To some extent, we may say :您贵庚是？

Just to clarify, "你几岁, Ni ji sui?" is informal and most appropriate for younger people, right? A formal way to ask (good for older people) is "您今年多大岁数, Nin jinnian duo da suishu?" This phrase means something like "What is your age this year?"

How old are you = ni ji sui = ni duo da =你几岁？=你多大？

I learned: 你今年多大了？