Arrogance can come from youth or old age, not just
one or the other. Assuming that somebody else envies your youth is arrogance of youth if it isn't based on
reality. For example, if a person that lived through the 80s (or 70s, or decades before) wouldn't trade their time during those years for youth, since youth would
mean they missed out on all that time and mostly just remember the
90's or 2000's, then they are not envious of anybody's youth. They had their own youth, and they are where they are now, and that is
fine with them, because they'd rather be themself than a younger version of themself, or you.
Arrogance can be there at any age. The kind of person that assumes someone else envies their youth must envy the youth of
people younger than themselves, and yet not every younger person would make or has made such a
narcissistic assumption about other
people.