Actually, this differs from individual to individual. I have met with different old men and I have learnt a lot from their experiences and perception towards life when they become old.
One of the things most of them regret about their youth is the lack of fulfilling their goals or purpose in life. The youth stage is one of the best stage you can achieve and accomplish your goals before it is too late. Some people never knew their purpose in life because they were just too busy doing other things during their youth stage and they end up regretting not fulfilling their purpose or aim because it is too late for them to do so.
Another thing some old people regret about their youth stage is their wayward lifestyle. Some people spent their youth life doing brutal things, living in a wayward manner and they end up regretting it when they get old. This is why you see old people giving you reasonable advise on living a good life at your youth stage.
Another thing some old people regret about their youth stage is their inability to manage time. Muhammed Ali once said "Life is made up of time and he who manages his time will surely enjoy life". Some old men and women wasted their time on irrelevant or unproductive things during their youth stage because they never knew each a time and days counts.
Lastly, majority of the old men and women do regret not fulfilling their spiritual or religious obligation towards God. They all know that they are gonna face death very soon which makes them get scared and scrutinize on how they spent their lives in relation to God. This is the worst of all regrets!
In conclusion, It is advisable we live our lives in such a way that we won't end up regretting in our old age just like some old people are doing presently.
Thanks for reading and I hope this helps.
People regret if they'd not lived to their potential, a person with average intelligence do not regret having a academic career but a person with intelligance does. People also regret about giving to much opinion about how other people think about themselves.
With time, most people are regretting fewer and fewer things they have done. They are regretting things they haven't.
A research showed people who have reached their goals in life, are dying tranquil, peaceful. Other who missed their goals are having hard psychic pains, depressions, remorse.
I think what we feel to have missed in our youth can be many things, individually different. Men can regret more professional failures, not having made a real career, missing to reach wealth, power money, Women are regretting not having married, staying without children. Many think they haven't married the right person.
And, many are blaming others, because seeing our own errors hurts a lot... "This pig is guilty of my failed life" - can you hear so often.
I found this today, interesting: