I am the adult child, rather than the parent here, I hope thats ok?
Funnily enough, it is something my dad and I have discussed at length.
He said him and my mum found my older brother most difficult between 13 and 17 because that was when he started to voice his opinions (or as dad put it, share what he read online) and these thoughts clashed so aggressively with my parents. I remember listening to them row, my brother going to university as mum puts it "was the best thing" for their relationship, now my dad and brother are so close!
For me, it was when I moved out, I've always had an extremely close friendship with my mum, I'd say when I got to 16, the respect was still there, but my parents stopped trying to parent me and more supported me and let me make my own choices and mistakes. Dad will tell anyone the most rebellious thing I ever did (slightly outing here) was go and get a tattoo on my 18th birthday... leaving home, I stopped asking for advice and opinions, mum said it was bittersweet to see because on one hand, it's what they had brought me up to do - be totally self sufficient and independent, but on the other side, we were close and I asked her thoughts on lots, and that just stopped one day. Since having dcs though, I see mum twice a week, my parents look after dc one day a week and I also meet up for lunch with her just the two of us weekly. And I talk to her most days.
Personally so far, I have found the toddler stage hardest, the teaching of right and wrong and trying to get children to comprehend it, can be so hard!!!