你多大了，一般对不同年龄的人都可以这样问，例如从小孩到老人。也可以问你多少岁了，你多大年龄了。对于老年人为了表示尊敬，应该说您（不是你）今年高寿啊。 你几岁了，一般只有问小孩的时候才用，因为‘几’表示的数量不是很多，不超过十个。

几 is usually for a single digit, just like you can use 十几 for a number between 11-19 or 几十 for 10, 20,... 90, so 几岁 is suitable for children.

You could ask a kid “你几岁了？” For an adult, you can say 你多大了？And 您多大了？is a courteous way of asking an elderly or senior person’s age. Also when asking an old person’s age, you can say 您高寿？高 means high, and 寿 means age. Together 高寿 means your venerable age.