I think the birthdate shown for JH was in the year 1980, making him thirty four years old at the time of Cooper's death.
Putting that aside, he's shown that he's capable of holding down a steady job, works in a field that requires brains, demonstrated that he can drop his son off at day care on a regular basis, smart enough to google pets in hot cars only days before Cooper was left in a car to bake to death. Presumably he carried on his so-called double life for months or even years without putting Cooper in danger at all. There is no reason to believe he's incapable of handling his life's responsibilities. I think it comes down to his own admission that he simply doesn't have a conscience. "Don't you have a conscience?" "Nope." His words.