It's at best just extremely rude to tell someone about their own life and how they should feel about it.
Not really. It's notional advice, offered in a spirit of kindness. You don't have to like it, but labeling it 'extremely rude at best' is absolutely infantile.
Using euphemism or calling it something better then it is is exactly as infantile as exaggerating. And if is lie too.
I have to disagree. My grandparents often have great perspectives on life situations that they readily share when relevant to the people around them. No one sees it as rude, and if they do, it’s usually because the advice hits a little too close to home. Like “when I went through my divorce…”
The truly insightful manage to prompt you to think without explicitly telling you to do so.
(And are sometimes wrong. People can in fact be living in a truly dismal present, whatever their age.)
"When I was your age, I didn't appreciate certain things so I'm pointing them out to you" is really different from "this is the best time of your life" which is a statement an external observer is just not equipped to make about someone else's life.