I think as soon as the child is out from under the parents care they are responsible for their own happiness. This is when they are able to spread their wings and figure out the world for themselves. Have their own responsibilities and realize they themselves are responsible for everything including their happiness.

It's possible it happens sooner but this is when I feel it truly is squarely on their shoulders. When they realize their parents did the best the could with what they knew. Provided they weren't abused in any way. I think we all go through some turmoil with our parents on an emotional and mental level that could be construed as abuse if you twist it in that fashion but most would agree that it wasn't really. I'm not talking about the unfortunate some that really were abused. Don't mistake my words.

My parents did not give me affection and encouragement. We were not allowed to question them or voice an opinion, particularly one in dissention. It has taken me forever to realize that I was made to feel unloved. All that stuff is stuck down deep. Michael Singer talks about releasing these energy blocks in The Untethered Soul. He has many YouTube videos, too. My point being that I was not aware of the core issue as the childhood issues were more subtle than outright abuse. Once you are aware of what is at the root, you can start reparenting and doing whatever else seems to help release the pain, to grow and become healthier. I don't think you can be really happy while you are depressed, and it has nothing to do with age. I also think parents have a responsibility to be there for there child in whatever way they are able, hopefully to express and share (unconditional) love.

some people blame their childhood for everything. Even if your childhood was awful, there comes a time when you have to own what is wrong with you and try to let things go. You are not hurting anyone else but yourself when you blame others'
just my opinion

Sometimes it is a choice.

age really differs, I would say when your peace is disturbed as a parent. If it is out of your control