The 15 year ago vintage class does have amazing returns.
However, assume for a second we can take different vintage years and say that return(t-20 to t-0) = return(t-20 to t-15)*return(t-15 to t-0).
Then all the money was made from t-15 to t-10 since the t-10 vintage class has basically zero returns and the t-20 returns are approximately the t-15 returns annualized over 20 years instead of 15
However, assume for a second we can take different vintage years and say that return(t-20 to t-0) = return(t-20 to t-15)*return(t-15 to t-0).
Then all the money was made from t-15 to t-10 since the t-10 vintage class has basically zero returns and the t-20 returns are approximately the t-15 returns annualized over 20 years instead of 15