You need to read up on your torts. Injury or harm done to a young person can result in a claim for compensation later in life. That's why paediatric insurance is so complicated.
It may be how UK tort law works, making it legally justifiable (or even legally mandatory), but the duration is still definitely stupid and wrong.
Well, "stupid and wrong" unless this is one of those things where you need to be a lawyer in the first place to understand all the moving parts that make no sense otherwise; but that's how bad it looks from here.