Question
AT WHAT AGE CAN YOU START FERTILIZING THE APPLE TREE

Answer
As long as you're old enough to hold a watering can, you can fertilize anything, Ernie.

Sorry, I could not resist.  Ask me how do you get to Carnegie Hall...

But seriously -- if your soil NEEDS a boost, amend it.  Apple Trees don't need to be fertilized if the Soil is right.  Early Trees can use a little more root-building Phosphorus.  Bonemeal is my favorite Phosphorus amendment -- but that's only if you need it.  Like I've said before, no amount of fertilizing will raise the ceiling of a plant genetically destined to grow a certain way.  No matter what those Miracle-gro photos of Tomatoes the size of a taxicab 'prove'.  Test your soil.  See what's missing.  Add that, and nothing else.

'tis the season to be jolly, Ernie.  Thanks for your question.  Any followups invited.  Got any jokes about Apple trees?