Our son is 3 1/2. Old enough for separation to be a negative factor in his life. Young enough to not fully grasp it so it's less damaged than when he's old enough to understand.
I should mention that early on in our relationship (<6months in), things were rocky, and I cheated on her with a number of friends of mine. Maybe part of my "validation" for her actions stems from my capacity to do something similar?
Fuck all, I don't know at this point.