In NL-nothing-NL is in fact about the only country in the whole of west-EUR that does not even offer lower priced junior/student seasons-but about any uni-student however gets a free (week or weekend) unlimited subscription as part of grant. You may want to look into InterRail 1 country BeNeLux if you intend to make a few longer traindays-its slightly cheaper for junior <27.
IN BE it is for ANYone-even those from out of the EU. It is either as goPass 1 for 1 trip (online only) OR as a 10-ride pass, old style cardboard, in which you write down details. It is a general rule that anyone from young age anywhere in BE always needs to have the ´eenzelvigheidskaart´ with her/him=ID and its common this will be asked for in doubt. But as a tourist you need to have that with you anyway.
AND-in most BE school holdiays there will even be GO-UNLimited, for a very low price (for 1 week or maybe longer)-but you must then load this on a personal MoBiB chipcard, cost 5€, can be made on the spot in major stations-they can even do the foto then (which in NL is also a crime to arrange for foreigners).
In LUX all transit is free. BUT note this GO-passes and all other BE cheap offers are ONLY valid till last real stop, NOT the border points. So if you plan to go there too: have to buy another single ticket from that last stop to LUX-only BE part will be charged. (mostly about 2,50).
In DE there is now till aug. the well known 9€ alllocal monthly=extreme overcrowding in many cases, as I also have experienced.
Yes, IN BE its for anyone from where-ever. As GOpass 1 for 1 trip or the old style cardboard pass for 10-in which you write down details.
In school hols thetre is usually also GO-UNlimited-but must be loaded on local MoBiB chipcard, cost 5€ for 5 yrs.
ALL cheap local tickets only valid till last real stop, not border tariff point, so pay extra for that short section.
You may want to look into InterRail 1 country=BeNeLux here, as it has lower price for juniors for some days in NL.
In LUX its free-but the short few KMs in BE till border thus must be paid for any trip in addition.