There are many kinds of regrets that people have and then feel trapped by within their current lives. That becomes more acute as you enter the middle years. But consider this: If you're able to feel regret to begin with, then that very feeling, that awareness, suggests that you're probably not the same person you were back then.
I wish I could delete the moments where I ever felt "old" because to think that now is just stupid. I saw the number 24 written on the cover of a book while on the train, and clearly thought these three things -- I am older than that number. I have not read that book. There is so much I have yet to do.
Sometimes, I wonder about the road not taken. I stop for a moment and imagine what my life would be if I'd taken the other path. I feel regret and sometimes an overwhelming sadness. I focus only on what I may have missed. I feel the void - as real as if that which I did not choose had been torn away from me.