Ds was about 2.5, I think, mind you many of the confirmed positives are things that he will be outgrowing anyway. (they are still getting to terms with the new foods)
I think the one and main advantage of York test is that it is a good place to start if you don't have any clue about what may be causing the problem, and you can order it even if your GP finds it difficult to believe that you need it (why I say that? because it took me 6 months and a private RAST test to convince the idiot GP that what DS had had was a reaction to peanuts... ) But as I said, it will be good to be prepared to corroborate the results with a more reliable test.
The test DS had was for intolerances (113), they said that they wouldn't do the allergy one for a child that young. However, and as I said, everything that scored high in DS's York Test was confirmed as a proper allergy by RAST and Skin Prick Test.
For the allergy test I think that you need to take a large sample of blood (may need a nurse to do it), for the intolernace one you get a little pack with a lancet and a tiny container with a rod similar in size to half a toothpick. You only need to hit the skin with the lancet and collect a few drops of blood with the little rod provider, send it back by post and they will send the report in a month with the contacts of the nutritionist whose consultation is included in the fee.