Correct. A person is guilty of an offence if he modifies an imitation firearm so that it becomes a realistic imitation firearm.
However, it shall be a defence in proceedings for [that offence] for the person charged with the offence to show that his conduct was for the purpose only of making the imitation firearm in question available for the organisation and holding of permitted activities for which public liability insurance is held in relation to liabilities to third parties arising from or in connection with the organisation and holding of those activities.(*cough* airsoft *cough*)
I can't see an age limit on that defence.
Let's see if a moderator takes the huff and censors these facts because it makes their feels feel uncomfortable.