As my understanding goes, very recent or current uniforms are not okay unless you have earned them in service. You would most likely be taken as a veteran. And it could be embarrassing to have people come up to shake your hand and thank you for a service you haven't rendered. I've used a WWII uniform, which is out of date, on many occasions and had this happen to me, and I would have to explain that I do not deserve their thanks since I haven't served in the military, but wear it as a reminder of the servicemen, and to honor my father, who had been in that war.
On the other hand, I have seen uniforms that could be taken as a recent or current uniform, but certain patches identify it as a costume from a particular fictional movie or television show, i.e. Stargate, are okay.
So I would guess recent or current uniforms with fictional patches are okay, these uniforms with actual non-fictional patches are not.