Could it be 'A Spell for Chameleon' by Piers Anthony? It's part of the Xanth series and has a lot of magical elements. There's a mirror involved too but it's kinda a long shot since that's more of a magical realm series than alternate dimension.

Maybe try 'The Dark Hills Divide' by Patrick Carman? It's got magical elements and secret passages that might be like the mirror you're describing. Came out in the early 2000s tho.

Omg, sounds so familiar! I remember a book from back then with a mirror and they had to solve riddles or something to save the other world? Ugh, it's on the tip of my tongue too now!

You're possibly recalling 'The Mirrorworld Series' by Cornelia Funke which starts with 'Reckless'. Has a male protagonist with a mirror that leads to a parallel world filled with fairy tale creatures and dangers. Even though it's not from the 90s, it has a strong old-world feel to it.

Lol, 'kid finds a magical mirror', you sure you didn't dream it up? Next you'll be asking about a book where the mirror tells you you're the prettiest of them all 😂

idk if thats it but u could also check out 'The Seventh Tower' series by Garth Nix, it's not exactly as u described but there's a lot of magical elements and different realms

Ooh, I think I know the book you're talking about! Was there also a thing with a prophecy and someone chasing the kid through both worlds? If so, it could be 'Through the Mirror' by John R. Dickinson. Not sure it's as old as the 90s though.

Hmm sounds like it could be one of the lesser-known titles. Reminds me slightly of 'The Mirror of Her Dreams' by Stephen R. Donaldson which is from the 80s but definitely has the magic mirror and alternate world thing going on. Maybe check that out?