So that probably sounds like a daft question to some, but bear with us.
When your car is first registered, the registration it is given is an identification code that does several things.
The two-letter regional identifier called the Local Memory Tag shows approximately where, geographically, the car was registered.
Date of first registration
It shows the date that the car was first registered. For practical purposes this defines the official age of the car.
It provides you, and the authorities, a way of identifying your car and telling it apart from all the others
For more details on the character codes used to show this information, please see how formats work.
When you buy a private registration and change your number plates, the last two of the things in the list above may no longer be shown by your plates. While you may, of course, purchase a number that bears the same date and location codes as the original number, this is vanishingly rare.
People usually buy registrations that are quite different - often with the intention to hide the age of their cars. While the car registrations regulations don't allow you to make a car look newer by showing a more recent date code, there is no ban on hiding the car's age altogether buy assigning a number in a format that has no year code.