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.