The legislation on the supply of real or imitation firearms to young people is very complicated. There are restrictions for actual firearms, depending upon the type of firearm, imitation firearms that look like firearms (whether they are realistic or unrealistic) and even firearms that are meant as toys.
In general, the legislation on supply of real firearms to minors is enforced by the Police, whereas, the legislation on supply of imitation firearms to minors is enforced by Trading Standards.
Put simply, a real firearm is one that fires a projectile (bullet or shot). Ammunition includes both the projectiles (bullets and shot) capable of being fired by a real firearm and also other projectiles such as grenades or bombs whether or not they are capable of being fired by a firearm.
An imitation firearm is one that resembles a real firearm, whether or not one that is capable of firing a projectile or being adapted to fire a projectile. An imitation firearm is realistic if, for all practical purposes, it is indistinguishable from a real firearm. In general, it is an offence to sell realistic imitation firearms at all, but there are some exceptions and, where those exceptions apply it is illegal to sell a realistic imitation firearm to a person under the age of eighteen.
An imitation firearm (but not a real firearm) is capable of being made unrealistic by making it in certain bright colours, transparent or to look like a pre-1870 revolver. These unrealistic imitation firearms (even if they are obviously intended to be toys) may not be sold to a person under the age of eighteen.
Some 'firearms' are considered so unrealistic that they would be incapable of being considered to be imitations of anything. Examples include 'futuristic space lasers' or 'super soaker water pistols'. This is a question of fact and degree for Magistrates to consider, so it may be risky to rely on these products being available to under 18's. However, equally, it is unlikely that Trading Standards Officers would consider it in the public interest to prosecute a retailer for selling a toy super soaker water pistol to a young person.