Ive think the latest guise on Merseyrail makes the units look a lot more modern than they are.Refurbishment of BR stock has given the impression of modernity to many passengers when refurbished stock was introduced some were under the impression it was a new train.
There is a bit of modern stock operating that feels an 'old train' such as the Corby 360's with tatty interiors. I would even dare to include the newer 800's on GWR.
But if you really want to include a really old train then the recently withdrawn former Island Line stock: That WAS old. 313's and 507/508 are in with a shout. Purely age of a train is deceptive like humans as somebody who is aged 25 may look far older and equally somebody who is 50 can look much younger.
To get a none enthusiast point of view, last time we went on over to Liverpool I asked the Mrs how old she thought the 507/8s are. She said around 10 years old!