It's all about continuous development and not how old an engine is. Unreal 4 is great in 2015, id Tech 3 with little development since 1999 feels like yesterday.
Unreal, ID Tech, CryEngine, etc. most modern game engines date back to the mid to late nineties. But where as e.g. the Unreal 4 engine has been in continuous development since 1995 and came originally with an editor coded in Visual Basic, one cannot compare the limited Unreal 1 of 1998 with Unreal 4 as of today. Sure many code lines are still unchanged and in Unreal 4 but especially the graphics and editor have changed a lot.
A modern Call of Duty (e.g. Ghosts) still use a heavily modified Heavy Metal F.A.K.K.² engine (Ritual's ÜberTools) which itself was a heavily modified id Tech 3 engine with the GtkRadiant editor. Infinity Ward added texture and map streaming in Modern Warfare 2. The id Tech 3 engine is now open source, but there is a big difference in the capabilities of what you can do with Infinity Ward Call of Duty engine and what you can do with id Tech 3 that has seen little development since then - also in the tool section.
GoldSource/Source/Source2(?) is one of the oldest engine still used in AAA games, based on Quake and Quake World. The Worldcraft/Hammer editor is dated in comparison to UnrealEd. Hammer editor for Source 2 engine seems to catch up.
Unreal, ID Tech, CryEngine, etc. most modern game engines date back to the mid to late nineties. But where as e.g. the Unreal 4 engine has been in continuous development since 1995 and came originally with an editor coded in Visual Basic, one cannot compare the limited Unreal 1 of 1998 with Unreal 4 as of today. Sure many code lines are still unchanged and in Unreal 4 but especially the graphics and editor have changed a lot.
A modern Call of Duty (e.g. Ghosts) still use a heavily modified Heavy Metal F.A.K.K.² engine (Ritual's ÜberTools) which itself was a heavily modified id Tech 3 engine with the GtkRadiant editor. Infinity Ward added texture and map streaming in Modern Warfare 2. The id Tech 3 engine is now open source, but there is a big difference in the capabilities of what you can do with Infinity Ward Call of Duty engine and what you can do with id Tech 3 that has seen little development since then - also in the tool section.
GoldSource/Source/Source2(?) is one of the oldest engine still used in AAA games, based on Quake and Quake World. The Worldcraft/Hammer editor is dated in comparison to UnrealEd. Hammer editor for Source 2 engine seems to catch up.