You can go alone at 17 - but for questions like this it’s best to check out the Nijmegen Vierdaagse website and read their regulations. This prevents any misremembering from any of us oldies.
As far as I can interpret the ACROMATIs, so long as you have completed Blue RM first then go on to do Nijmegen yourself, you’ll qualify for the Nijmegen badge upon application.
There is also a separate Gold Road Marching badge that’s available if you can’t get into Nijmegen (it’s a very popular event), for completing things like Dodentocht, Izjer or the Cadet Forces March.