I wish I'd listened to my grandparents when they were offering advice and help.
I also wish that I'd learned the carpentry and fishing skills that my Mum's father had, and was keen to pass on to me (I have his fishing gear but need to learn how to use it properly). I wish I'd learned my Dad's father's skills an a precision engineer.
However, it's not all bad - I have learned some excellent knowledge and skills from my parents and grandparents. I learned that you shut up, work hard and keep on getting up when life knocks you down. I learned that life owes you nothing so it's up to you to work hard and try to be a good person that others will value and respect. I learned that you get out what you put in, that having a dog is important, that doing half a job is unpardonable, that you face your responsibilities, that you look after others for their sake rather than your own, that throwing things away rather than repairing them is foolhardy, that we have it so much easier than others, that being outside under the sky is best, that you support your family come hell or high water, that you can learn something from every person you ever meet.
Yes, I missed out on a lot because I was young and knew it all already, but I think that, even if I didn't realise it at the time, I learned far more than I missed.