Apparently the answer to this question is completely subjective. According to Layton, it is perfectly ok for it change according to the situation.
When he is clingy and acting like a baby – ask how old he is and he’ll tell you – Zero
When it is time for school in the morning – ask how old he is and he’ll tell you – Five
On Wednesday night when it is time for Dallin to go to Cub Scouts – ask how old he is and he’ll tell you – Eight
Any other time – ask how old he is and he’ll be honest and tell you – Three
I’m afraid to ask how old he is when I’m driving down the road – I’m pretty sure he’ll tell me – Sixteen – and that would be way too much for me to handle :)