I will say that no, there is never a time to punish during TT as it would be counter-productive. You don't want him to get scared of going to the toilet

My two boys were TTed at 4 and 3 and a 1/2. They weren't interested any earlier so I would say at 3 and a 1/2 with only random accidents is good (for me anyway).

Have you tried getting him to help clean up?

Oh, and TT wasn't my hardest parenting challenge. I was lucky and it went very smoothly. My hardest has been brushing teeth and washing hair. It is still a nightmare.