Young people： 你多大了？你几岁了？ Old people：请问你贵庚（gui geng）？你高寿（gaoshou）？

It depends on who you are asking. If you're asking old people, it's not polite to say "你几岁了？". Instead it's better to say "您多大年纪了？" or "您高寿？". If you're asking children, I think "你几岁了？" and "你多大了？" are both OK. As for adults or teenagers, I think "你多大了？" is better. So my opinion is that "你几岁了？" should only used with children.

Polite way would be : 請問你几岁?