I'm younger than you, so 20 years goes probably too far back. That would be basically anything beyond my teenage years.
The boards show I joined March 2009, so maybe something around that time. Or 2007 when I got married and suddenly had a lot less time alone. That would kind of, sort of, align me with your cutoff point age-wise.
So these are bands I hadn't listened to prior to around that time in my life:
Axel Rudi PellMichael Schenker GroupAyreon - I jumped on board in 2013 when I bought the Universal Migrator 2CD. I was already a fan of Star One though.
Hollow / Binary Creed - I remember posting here about buying the first two Hollow albums.
Mystic Prophecy - I've become a massive fan. They are one of my all-time favourites now. I only knew Valley's Eve prior to joining the boards.
ExtremeHelstarMasterplan - I bought a bunch of their CDs from a board member here.
Morgana Lefay - I think it was 2018 or 2019 I finally got into these guys.
Powergod - they are very good. There was a thread about them here.
Metalium - got into them because of Chris Caffery
Powerwolf - my favourite "newer" band.
BrainstormSaigon KickTarot - got their entire discography quickly after hearing the first album
TeslaThresholdTwisted Tower DireUFO - I really love these guys now.
VoivodWuthering Heights / Civil War - I discovered Nils Patrik Johansson not long ago.
Is that 20 already? I lost count. Everything else I like I've already listened to earlier.
I could almost add one of my all-time favourites Rage because I only had 1 of their CD's before the Great Plunge a decade or so ago when I purchased everything by them. But I won't. I actually already listed 21 bands. And I forgot Winters Bane. Listening to Powergod got me into the Winters Bane vocalist. That can be one entry