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In my experience, it's often the new/changed code that is the most interesting. It's often the code that is causing some unexpected bug. 'No one sews a patch of unshrunk cloth on an old garment. Otherwise, the new patch pulls away from the old cloth, and a worse tear is made'

Physical things often become darker with age due to dirt or corrosion. Examples are wood and metal, or an old vs. a new wall painted in the same color. They all become darker over time. Therefore I think it't more intuitive to think of darker things as older.

You are ignoring survivor bias.

Old code is mostly good code. It's new code that has varying quality.