And besides a drivers licence your bike needs to have a licence plate and has to be insured. Like stated above, only on a closed cross circuit all of this is not needed, but in that case you need a trailer to get there and those circuits are not for free (and not open every day).
The good news is that there is plenty of trials around Eindhoven (roughly everywhere below the line Breda-Tilburg-Eindhoven-Venlo). The bad news is that police and forrest rangers (if that is the correct word ..) are patrolling quite a lot so be aware where to drive (avoid Belgium at all, fines are higher overthere).
If you plan for the circuits I have not much advice on a bike (by the way, there are much more circuits, also in villages around Eindhoven). If you plan for off road, you'll notice a bit more heavy 4-strokes are the most popular (400cc and up roughly).
I wonder what brought you here, for off road riding I would have stayed down under
gr Jos (also from Eindhoven)