Yes you have to be over 18 to buy a realistic imitation firearm (RIF) or a two tone. However your parents can buy you a two tone. If any of your parents have UKARA then they can get a RIF for you. I'm under 18 but my dad has UKARA so I have RIF's.
I reckon this ukara thing is just a ''seen to be doing something'' law, i can buy a .177 co2 multi shot replica of a 1911, beretta 92fs, sig p226 and others without any trouble.