Source engine itself is at least 16 years old, and has pretty direct lineage to the original 21 year old Quake engine (Quake (-> Quake II) -> GoldSrc -> Source

Source engine itself is at least 16 years old, and has pretty direct lineage to the original 21 year old Quake engine (Quake (-> Quake II) -> GoldSrc -> Source). I would be more surprised if there weren't lots of RCEs in it.