Your oldest Light Novel and Manga
My first light novel was Rising of the Shield Hero Vol 1. That's pretty easy to remember since it was only a couple of months ago. I can't really remember my first volume of Manga. That was 15+ years ago and I easily read hundreds of volumes since then (possible more than 1000) and not everything was in volume format (like I read all the Bleach chapters recently, but that was on the Shonen Jump app).
Oldest light novel? Well, arguably the one that started the whole trend really... Boogiepop Phantom! Boogiepop and Others, specifically.
Oldest manga is trickier though. Technically it would be my oversized flopped Gunsmith Cats manga (Bonnie and Clyde being the first) but I'm pretty sure I actually started reading Love Hina first, in 2002. If there was any manga I bought before that, I no longer have them.
My Oldest manga should be a 20+ years old Doraemon: Petualangan Nobita di Negeri Sihir or Doraemon: Nobita's New Great Adventure into the Underworld, Last time I saw it it already lost its cover don't know where it is now tho.
while I've been reading manga from manga rental ever since I knew about it, that manga was the first ever manga I bought.
as for LN, with my country being so under the radar of such literature work,
my first physical LN was Penguin Summer its (2014), my 1st JP LN was Seirei Gensouki vol.5 (2016), then my 1st EN was Re:Zero (2016)
I just bought them for collection tho.