When someone asks what age you are, he/she will say: Dè an aois a tha thu?
When you ask his/her age in return, you put more emphasis on the question, just like you do in English: What age are you?
You do this by using fhèin.
Dè an aois a tha thu fhèin?
Also, remember to swap thu for sibh, if you are being polite or talking to more than one person.
Sometimes you will hear people use the words: bliadhna a dh’aois after their age.
Mar eisimpleir: Tha mi fichead bliadhna a dh’aois. I am twenty years old.