I joined Asobo Studio[1] for an internship back in 2010, and some code there was already nearly 15 years old as they grabbed the remains of Kalisto[2] including the engine/editor. That editor (Pawap) is still used to this day, although the engine (Zouna) has been rewritten (but uses the same data backend as the old code, albeit heavily updated too). This was probably the oldest codebase I worked on at the time.

[1] https://en.wikipedia.org/wiki/Asobo_Studio [2] https://en.wikipedia.org/wiki/Kalisto_Entertainment