True, they can choose to apply whatever they want. BUT... the easyest to apply are the Replica laws and that leaves you to argue it with the police.
Right or wrong, they can do that. The Replica laws have teeth. It's about the only clear thing out there.
Not that what I say matters, but I had that discussion with a police officer in the past and that's the information they would use. Because they have the discretion to do it. In the field, you dont have time to figure out loopholes, you use what works.
As you just said, the younger you are the worse it gets. The Replica laws are clear about who can own/use one, and where too. The only problem here is underage people who refuse to follow basic rules, common-sense, and applied laws because they are too mature for those things.
Waiting a few months or 2 years to be legally responsible for your self is too hard for their maturity to cope with.