Well RVSB, there are mixed opinions in this as the Hindustan Times says that diesels above 10 and petrol above 15 years are banned. I think that the central government intervenes and bans the commercial vehicles instead of not allowing the private vehicles to ply on Delhi roads. America has stricter checks and still a lot of old vehicles like Mustangs / Chargers / Camaros of the 1960 and 1970s still rule the roads. The 1950's Corvettes fetch a fortune to the owners and more and more people preserve them. The Duesenbergs can fetch you a cool million and was out of production in 1950's if I'm not wrong. The 1950's Ferraris are also collectible. But at no point of time, America imposes a silly ban on the Vintage Cars and bikes but feel proud of them. They also follow Strict Checks before allowing them on roads rather than dumping them in scrapyards.
The rules at least need to have some more points like,
Commercial vehicles above 10 years can change their (New) engine/Transmission if they are continuing with the same Vehicle.
Private vehicles need to produce the fitness certificate after 10 years.