あなたの年齢は？って英語でなんて言うの？

These are phrases which have the same meaning. The second is a more polite way of saying it especially when you do not know the person. The first is used wherever there are English speaking people and most will respond well to it however if you have never met them then its best to use the second phrase.

When asking someone how old they are this is also called their 'age so you could ask 'how old are you?' or 'what is your age?' these would both give you the answer you want

If you would like to find out how old somebody is, you can do so by saying the following: "How old are you?" "Would you mind telling me your age?"

All of the following answers are an acceptable way to ask someone's age, whether in a casual or formal situation.

You wish to know someone's age and so you ask them using one of the above examples questions.

The most two most common way of asking for someones age would be to use any of the above.

Some people age really well and they look very young for their age. You have two children! What is your age? You look very young. How old are you?