A 20 year reunion seems to me possibly prickly.
You think you are supposed to have achieved something 20 years after.
10 year reunion is at a more carefreee time still.
30 year, you have had a lot more life experience, and maybe are more at peace with who you are.
I attended my 20 year reunion, and the major feeling for me was one of great joy to see my old classmates. I think we are a lot more forgiving at 37ish than we give credit for.