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Explanation: Depending on the context, you may also translate for something like "pensas que tenho a tua idade?", but that, of course, depends a lot on the context.
However, the I think the "correct" translation is the above mentioned.
Explanation: Because it a casual way to communicate it is implied ,I mean between the lines,
Quantos anos achas que tenho?
Or also could mean my emotinal age or a response to a situation like this:
Someone tries to prohibit something and then the person ironically says/asks
How old am I?
Got the point?