I disagree, as Cod and Twiglett know, with them. I think you can be 'gifted', as a child, as reading. That would be something like reading adult books with full comprehension at 7 or 8. The 'gifted' bit is the understanding and interpretation, not the decoding, which is relatively straightforward.
I think, though, people should consider accepting that the word 'gifted ' is changing meaning in the UK, along US lines (just as I have to accept that the normal spelling for 'pooh' has become 'poo'). In the US, 'gifted' has been used for a long time to mean the brightest 3-5% and gifted classes, gifted pull-outs and gifted streams are all the norm. Not all these children are mind-blowingly brilliant.
Reading ages are pretty meaningless, in my opinion - types of test and questions vary massively. Having said that, in DS1's Y3 class the top 25% or so all had reading ages above 11. The teacher may be basing her judgement not on a score but on a general observation of his ability.
Anyway, I would encourage him as everyone else has said and be please that he enjoys reading and is good at it.