I've seen plenty of code bases become less reliable over time. I once took over a project that had been going on for 10+ years. Reliability was terrible. The first thing I did was stop all changes, and only allow critical blockers to be fixed. That eventually made it stable enough to be useful.
Reliability takes more that time. It takes competence and attention to detail.
Mature does not imply old; it means feature-stable, proven and understood.
An example I know, Canon's pro-level cameras are mature on the day that they go on sale to the public because they have been tested around the world for a year or more to verify and debug the design. That's part of their appeal, users know that they're putting $8000 into a mature camera that will get the job done, though with fewer features and baubles than a Sony.
Reliability takes more that time. It takes competence and attention to detail.