I wonder about this too - the generational divide.
I find it strange that I can have on the one have the close bond I have with my kids (16, 18 and almost 21), and on the other hand can feel so separate from young people (admittedly more likely to be between 25 and let’s say 35) when out and about.
But I disagree with the notion of a hay day - as if the time I am living now (at 53) is somehow less important or when you think about the amazing fact that we all made it on to the planet, wondrous.
My son asks me about what things were like in “my day” but aside from telling him yes we did have x, y or z (!), I also say that just like for everyone else, now is my day.
I think the fact that our society is so ageist (who wants to face their own mortality?) really doesn’t help, but having been through a phase of wondering how I am relevant, and also currently feeling somewhat invisible, I also think that this is just a different phase. New and different things matter. The hardest thing for me to accept is that having been through the ending of what was an emotionally abusive marriage, it looks like I am not going to be in a relationship again - so I won’t have known what a healthy and loving intimate relationship is like. But it’s all relative and there are people all over the world suffering awful things so I am going to feel grateful instead.
To answer the OP, I also feel nostalgia and sadness (especially as my Mum died 16 years ago), but I try to block it out as it can feel very painful.
I am trying to stop living in terms of “things will be better when I achieve this or that” but that’s hard as well because circumstances mean that I am currently stuck in a job I am not mad about and I don’t have the money to do what I really want to do….
The other thing I find hard is living far away from all my extended family - who in any case are half in the UK and half abroad. Having lost friendships over the lockdowns, and being out of the phase of having lots of other parent friends, it does feel like a lonelier time. Not currently sure how to change this.