oh this brings me back to when i used to work in banking, i got the privilege to do maintenance on a "simple" piece that was kind of a cornerstone for one of the nightly tasks, it was created about 5 years before I was born
I had a similar experience at my last job, we had several Oracle databases which held decades worth of not just data but core business logic that was critical to the business.
Within a few months of starting I found myself modifying a stored procedure that had been written when I (in my 30s) was in primary school.