"Same as under 16s not being allowed to buy disposable razors."
The Offensive Weapons Act 1996 makes it an offence to sell to anyone under the age of 18 any knife, knife blade or razor blade, axe or any other article which has a blade or which is sharply pointed. That appears to include wooden skewers, pointed sticks, needles, knitting needles screwdrivers, chisels etc. Because of course the first thing anyone aged 17 years and 364 days would try to do is to stab someone as soon as they got their hands on a sharp object.
Disposable razors don't seem to be permitted but razor blade cartridges are if less than 2mm of blade is exposed. Single and double edged safety razors are banned.