Best European cities to visit as a youngish single person?

I’m about to turn 30 and have never really experienced any European nightlife, hostel culture, or any of the things that young, single people do when visiting there. My only experiences in Europe are Ireland with my family, and Madrid/Barcelona with a former partner. From what I’ve read, Barcelona has some of the best nightlife in the world, but I didn’t see any of it when I was there. We were back in our hotel by 11pm every night.

What are some places you recommend that I go before I get too old? Like if it’s already a stretch to go party somewhere as a 30 year old, I want to go to that place now before it becomes unacceptable to do it as a 40 year old. I didn’t start traveling until the past few years, since I didn’t have the money, knowledge, or maturity to do so in my early 20s, and now I’m feeling like I missed a stage of life. I guess I’m having a 1/3 life crisis and trying make up for lost time, but I figured it’s better late than never.