I've had cognitive decline over the past 6 years. I turn 41 this year. It's strongly (maybe solely) due to stress, anxiety, depression, migraines, and insomnia. This entire clusterfuck started 6 years ago because, while I had the skills to excel in school and career, I was sorely lacking in the skills to be a father, husband, and homeowner. When I got married and we began having children, my decline began. But it was slow enough and normal enough at first (new parents often don't get enough sleep, they often have extra stress and anxiety, etc) that I did nothing to combat it. It wasn't until 2021 with all the added awfulness of 18 months of a global pandemic that it become obvious things were unsustainable. I was barely able to function at work, I was being put on a PIP, and I was almost completely absent from my family. When I was with my family I was irritable, angry, constantly complaining.
I got on antidepressants and that helped, though it came with it's own set of problems. I started seeing counselors, reading self-help books, I went through 6 months of cognitive behavioral therapy for insomnia (CBTi), working with a neurologist to get migraines under control, seeking help wherever I could. It has been a slow process but I'm doing a lot better. I'm still nowhere near where I was cognitively. In fact I don't know that has improved much at all. And it's only been in the past year that I've began to understand my decline as a result of jumping into marriage, fatherhood, and home ownership without the necessary skills to handle them. I'm hopeful things will continue to improve, I've learned an enormous amount about life, fatherhood, marriage, love, forgiveness, hope, and priorities.
If you are a young parent, or considering being a parent soon, work on yourself. Ensure you have the skills you need or your life (and the lives of those near to you) will become a bag of utter despair filled with shit.