You can certainly use it. You can use it in all NS trains, e.g. from Schiphol to Lelystad.

When you buy the ticket, you will have to choose the travel date. After you've bought it, you have to load it in the NS app or print it on A4 paper. You don't have to do anything further to validate it. You can use the barcode to open the gates (if there are any).

You can read more about it on https://www.ns.nl/en/featured/jongeren-dagkaart.html

You can travel only on NS-trains (most are yellow+blue!).  Valid ONLY off-peak (mo-fr) and for any distance. In outer corners of Nederland some raillines are not served by NS but other operators-but not for the route you mention.

Hi I wanted to know if I can travel from frankfurt to den haag centraal with this ticket.

No  way, it is only valid in the Netherlands. Frankfurt is in Germany. You can get a card for use in Germany for nine euro’s form Frankfurt to the last station  before the Dutch border. Not valid in IC of ICE trains, you need to travel with regional trains. It takes more time but it is extremely cheap. Frankfurt-Kaldenkirchen near Venlo takes a little less than six hours.

Of course not.