I have yet to find that age.
I think maybe it starts out the least stressful and continues on a downward road from there.
After all, while they are still inside of us, we have little to do to protect them, when they are born we have a little more to do to protect them. Still relatively stress free, really. It's after they start crawling and then walking that all the stress starts. I don't know if it every lessens.