I’m sorry but if your kid who has graduated with a master’s degree and has a $150k job is “haunted by the memory of watching his peers graduate before him” such that “He would’ve much rather graduated with his peers and ended up with a 50k job” there’s more going on here beyond starting school at age 4. If he is as he says “traumatized”, and would have equally fixated on his peers getting their driver’s licences before him, it appears that there’s a bigger issue here and that he could benefit from counselling to help with his tendency towards distorted thought patterns.
One of my kids son has a late November birthday. Where we are the cut off date for school is December 31 so he was 3 when he started Jr. Kindergarten. He graduated university this year. Being a boy with a late birthday, he did struggle a bit emotionally initially, especially as he ended up being identified as having a minor learning disability which was quite frustrating to him when he was younger and which coloured his view of school. Eventually he grew out of the emotional volatility and while he’s still very sensitive, he’s learned how to cope and manage his emotions.
As to the driver’s licence, he turned 16 during Covid which delayed him getting his licence until he was 19.
If I could go back and do it again would I have held him back? Maybe, but who knows if it would have made any difference. In any case he managed to graduate from a very challenging program and is now employed and earning a good salary and is very happy and well adjusted.
Ultimately you can’t live your life with “what if’s” and looking over your shoulder to the past and with the fear of missing out. As parents we do the best we can for our children and we will make mistakes along the way. Part of our role is to help our children develop resiliency and the skills to deal with challenges. Not holding your kid back a year from starting school is hardly something that you should have recriminations over. If your kid is still struggling with dealing with the consequences and you’re so fixated on it having ruined their lives, I think that you both could use some professional mental health counselling.