From the guide to the firearms act Nov 2022.
A bit confusing as 7.11 seems to indicate it's been 18 since 1968, but 7.13 says it only became an offence for the seller in 2010
Air weapons in England and Wales
7.11
Section 22(1) of the 1968 Act makes it an offence for a person under the age of
eighteen to purchase or hire any firearm or ammunition, and this will include air
weapons and ammunition for air weapons.
7.12
Section 22(4) of the 1968 Act makes it an offence for a young person under the age
of eighteen to have with them an air weapon or ammunition for an air weapon, other
than when they are:
i. supervised by someone aged twenty-one or over (section 23(1));
ii. a member of an approved club while engaged as such in connection with
target shooting (section 23(2)(a));
iii. at a shooting gallery where only air weapons or miniature rifles not
exceeding .23 inch calibre are used (section 23(2)(b)); or
iv. aged fourteen or over and on private premises with the consent of the
occupier (section 23(3)).
7.13
Section 24(1) of the 1968 Act was substituted by the Firearms (Amendment)
Regulations 2010 (SI 2010/1759) with effect from 28 July 2010 and the revised
provision makes it an offence to sell or let on hire any firearm or ammunition to a
person under the age of eighteen. This includes air weapons. Under section 24(4) of
the 1968 Act, it is an offence to make a gift, or part with possession, of an air
weapon or ammunition for an air weapon to a person under the age of eighteen.
7.14
Section 24ZA of the 1968 Act makes it an offence for a person in possession of an
air weapon to fail to take reasonable precautions to prevent any person under the
age of eighteen years from having the weapon with him. This does not apply where
the person under the age of eighteen years is permitted to have an air weapon with
them under one of the exceptions set out in section 23 of the 1968 Act. The
provisions of section 58(2) of the 1968 Act mean that the offence in section 24ZA
does not apply to an antique air weapon held as a curiosity or ornament (see
Chapter 8).
A defence to the offence in section 24ZA is provided where a person can show he
had reasonable grounds for believing the other person to be aged eighteen or over