(I apologize for double post/spam.)
If you don't enjoy your life anymore, or you just feel so dead/empty/unsatisfied if you don't like harsh words, it's completely okay to miss your past, or to wish you lived in some past years you didn't. The only thing that isn't okay is completely living in the past, not trying out new things, not searching new things, not finding good things in bad situations (depends tho, i am kinda an optimist but there are some situations where i just cannot think positive), this is unhealthy and it makes you feel even less happier. (Sorry if this is hypocritical and kinda obvious)
If you want to live in the 90's, 2000's or any other decade, that's okay, just don't insult people who have different interests. We all want to go to the past just to feel alive again, but we can feel alive in the present too. Maybe for some people, it's difficult, but it all depends. Everyone reacts to things differently. Some people don't feel alive not because they want to not feel alive, (like most of times, you don't feel sad because you want to, you feel sad because something bad happened) but because this new era feels off/strange for them, and for some people, life in modern times just doesn't feel the same like before, and they lose interest, and become empty on the inside. Or just a bit unamused. There are some people who do not enjoy modern life, but still try to find happiness in it. It all depends on person's life and experiences.

Tl;dr: If your life feels unfulfilled, or you just do not enjoy the modern life, it's ok to miss the past you experienced, or to feel like you would fit in some era you didn't live in. It's a normal thing, methinks.