As scotcher says: 1.You are not old at all! is more emphatic than 2. You are not at all old!

With normal pronounciation the main stress of sentence 1. falls on the adverbial 'at all' pointing to the complete falseness of the idea: You are not old at all. In fact, you are very young.

In sentence 2. the word 'old' is emphasised so the sense could be: You may be a little eccentric/grumpy/in poor health but you are not all old.

I'm sure you know that in many languages word order can be used to express shades of meaning, with the beginning and end of the sentence being the strongest positions.