At the moment under 21s are unable to buy simple household items because they are considered too immature to buy them. At 21 years you can be trusted to drive, live independantly and get married however currently you are not trusted to buy a breadknife to go in your kitchen. Personally i was astounded when at the age of 20 i was asked for ID and subsequently turned away when trying to buy a wallpaper scraper because i was told it had a blade.
This law is ridiculous as anyone who needs these everyday items will get them anyway through friends or family over 21, however it causes a great deal more hassle than there needs to be. I cannot see how refusing the sale of such items can in any way be reducing knife crime in the UK.