I know that within an informal/familiar setting, people often leave out particles. Whether or not this is grammatically correct I'm not positive, but for the sake of this question, I'll say that it's ...
I think the sentences: 何歳ですか？ いくつですか？ 年齢は？ お年は？ are all sentences that ask "How old are you?" How are they different? What form is most polite?