If you have the materials, it doesn't hurt to apply. We have found it very worthwhile.

My daughter had been grade skipped when we applied. I think they do consider kids with their grade (if you read the details for qualifications, some things seem to be based on age, like achievement test scores, but others are based on grade). It's hard to really clearly say what is 2-3 grades ahead, though, because different schools differ. If your child is doing very advanced work, it is worth a try.