Nijmegen is the oldest city in The Netherlands with an asterisk.
At different times, two other cities have been called so.
For now, Nijmegen appears to have been the earliest proper city of the three candidates.
* Here's the asterisk:
A study in 2005 of some Roman coin proved soldiers visited in 19 B.C. The "Pillar of the Gods" (godenpijler) found in Nijmegen in 1980 dates from A.D. 17, during the caesarship of Tiberius. Rome confered upon Nijmegen a charter in A.D. 100, and it was certainly an important city in the empire.
But Nijmegen has been abandoned at least once. When germanic tribes north of the great rivers pushed back the empire in the failed Batavian uprising in 69-70, they burned Nijmegen (Oppidum Batavorum) and the city seems to have been unpopulated for years.
Albert Delahaye, city archivist from 1946 to 1957, insisted that Nijmegen was uninhabited for hundreds of years during the middle ages a time when popular history maintains that Charlemagne kept a palace there. This claim is not considered serious by professional historians.
Voorburg is not much of a city now it's more like a suburb of Den Haag. In any case, its "mijlpaal" (Roman roadside milestone) is not as old as the "godenpijler," so Nijmegen beats Voorburg for age, in archaeology thus far excavated.
Underground work in the year 2007 in Maastricht revealed a Celtic road, potentially 2500 years old. Evidence of older civilization, yes but Maastricht was never a city (municipium) during the Roman years.