Oops! My typo!

5.
- Did you punish him for that?
- No. I remember how young he was then that I had to excuse him.
(Is "then that" here a phrase shares the same meaning with "so that"?)

Thanks!
But I wonder which part of speech does "then" then belong to and its meaning?
And what is a " a much sentence "?

I meant to say "a much better sentence"! Sorry for the confusion. I think I've generated more than I've clarified.

Camel