"What age are you?"
Translation:Cén aois thú?
I have never heard it pronounced this way. There's lots of hits for both ways in a google search, e.g. http://www.forvo.com/word/c%C3%A9n_aois_th%C3%BA%3F/
These kids pronounce it your way (Ulster accent): https://www.youtube.com/watch?v=qQGYBR8knK4
An bhfuil sé rud réigiúnach?
Pronunciation doesn’t always follow spelling; for example, féin is often pronounced héin, and is sometimes dialectally spelled fhéin to reflect that pronunciation. I suspect that the pronunciation of tú as hú in this question reflects a similar situation, and the spelling thú is likely used to match such a pronunciation. Grammatically, tú is the spelling for the nominative case.
EDIT #1: It could be that thú is used to reflect the copula that’s hidden in cén — if that’s the case, then thú would be the proper form.
EDIT #2: Yes, that’s the case. Your initial thought was right, and my initial response was wrong — thú should be used in this sentence because of the copula hiding in cén.
To add to what Scilling has said, the Caighdeán Oifigiúil says: 'It is customary to lenite the personal pronoun tú when it is the object of a verb, e.g. báfar thú, molaim thú, and in some copular expressions, e.g. mo ghraidhin thú '.
I don’t remember which skill(s) here covered pronomial usage. You could look at the Gramadach na Gaeilge page on personal pronouns. Tú is actually wrong here.
Can't reply to your last post, so I thank you on the previous one. Some copula pronouns are introduced in Basics 1 exercises but thú was not explicitly explained anywhere in tips and notes so far (I am currently in Adj.).
I used tú in my original answer, which was accepted, but your earlier comment made me think maybe it's preferable to use thú with the copula.
There a couple of ways to answer that. One way is to point out that you never have a noun immediately after cad, and it might be better to think of cad as "what is" rather than just "what". cén can sometimes be thought of as "which", but in this case, the cén of cén aois thú? is usually rendered as "what" in English.