As a Bubbe (grandmother) myself I'd answer it this way:
"Do you remember last year when it was almost your ___th birthday? One month or so before the big day we started saying that you were almost __(the next age). We estimated and to do this we rounded the number up to the next year; the coming year. When you take a look at the number 33.6 what so you see first? You see the 33. And you know that it will be 33 until it turns 34, just like you really stayed ___ until you turned ___. But somewhere after the middle point; after 33 and 1/2 which we write 33.5 the number changes and starts calling itself (to anyone who will listen to a number) saying "I'm 34....almost." When it matter that we're very, very accurate we say you are __ year, 11 months and 29 days old. And when it matters very, very much we say a number like this one is 33.657890564677. When it matters a little less we say you are __ and 11 months old and, in the same way we say this number is 33.6. It's short. It's easy. Nut when it hardly matters at we say you're almost ___ and we make it really easy and say that 33.6 is "34."
When there is something new you, or he. have never seen before, begin with what you do know.
Hope that helps.