I think its luck of the draw a lot of the time!
My mini dwarf lop was still lively and ridiculous at 5 years old...he died of an unknown illness, not old age though...
I've just been told my german lop is showing signs of aging now shes 5/6, shes just starting to slow down a bit and has cataracts developing...my vet said she doesnt see many large bunnies past 7years old sadly.