Mark Twain is erroneously credited with a quote that at 14 his "father was so ignorant I could scarcely stand to have him around. When I was 21 I was amazed at how much the old man had learned in seven years." That's true as far as it goes, but it leaves out what happens later.
I have often used a similar quote: "Don't criticise your parents until your own children are grown." There is more in that than the mere cynicism of "walk a mile in my moccasins," and "don't be a smartass," which is how we usually interpret these thoughts.
My children are grown now, two out of four in adulthood with the other two darn close. I was quite critical of my parents when I was younger, more often behind their backs than to their faces. The issue was complicated by having 3 fathers: a biological father who my mother divorced in 1959, her boyfriend from 1961-1965, and my stepfather who she married in 1966. I had contact with my bio dad after I turned 18 and continued it until he died a few years ago. My mother died in 2000, my stepfather remains alive and has insisted we no longer contact him.
From about 25 to 45, I gradually and grudgingly increased my admiration for all my parents, much as pseudoTwain suggested. At about that point I thought I had made peace with who they were and who I was, the good and the bad, and had few open resentments (though certain things would still easily irk me). But since that time, the pendulum has switched back some. Adopting two teenage boys, which echoed the blending of my own family when I was 13 and my brother 10, I came to see clearly exactly what should be expected of you when you are the adult in a difficult situation with a child.
I decided that not only my stepfather, but my mother, did not meet their responsibilities. I have less energy about this than I did in my teens and twenties, and what I assign blame for is different, though there is considerable overlap. I don't often think about it (is once/month often?). I don't see anything about the situation which requires intervention or fixing. It just is.
Perhaps there is another sea-change yet to come, and in 10 or 20 years I will give them higher marks again. I am 53. Right now I give them a mixed report, with a few failing grades amongst the good ones.