I do not know what those formally trained in child psychology believe, but from my untrained (but somewhat experienced) perspective, the critical years for a parent to set the foundation for their children is when the kids are between 6 and 12 (give or take a year on either side based on the child).
This belief dictated several things for me, and two in particular - availability and approach. It affected my personal availability decisions, to the extent I could control or influence. Meaning, if I was going to miss a significant segment of my daughter's life based on having to physically leave (as a military officer, this equated to deployment or geographical separation), or not be available due to some school or training, the time I tried to avoid or minimize were those critical shaping years. Therefore, I tried to miss the earlier years (before they turned 6) or when they were teenagers. Because I wanted to be there to guide and teach them for the 7-8 years between 5 and roughly 13.
Regarding approach, I would be aggressive in finding time the girls and I could spend together. I wanted to engage them in as many life areas as often as possible during these years. Thus I spent as much quality time in the highest quantity as possible. I focused a lot on their development because, as the title states, these are critical shaping years.
Once you decide which years are most critical, you should find ways to maximize your availability. Similarly, you have to find time to spend with each of your children. I targeted the 6-12 range because I found the girls still considered it "cool" to hang with daddy, to listen to what I said and respect my role in their life. It was also a phase where they could understand a fair amount of what I was talking about, and what they did not understand I knew I would be able to re-explain later.
Lastly, regardless of the age range, you have to figure out what to teach them during those years. I have some suggestions, and will gladly offer them...but just not in this post 😊.