The position of dijkgraaf seems to have the same age-restriction as mayors do. According to Wikipedia, the dijkgraaf chairs "both the legislative and executive council, while having both ceremonial and representational roles as well as his own portfolios." Historically, this position seems to have been more political, from the same Wikipedia link:
In medieval times and earlier however, the water boards were the same as municipal, and since it was a country of duchies, the Water board (Waterschap) was in governmental terms the equivalent of a city (Stad), and thus also the highest form of government.
From the procedure on appointing chairs of the water board (in Dutch, PDF):
Als de voorzitter van een waterschap de leeftijd van 70 jaar
bereikt, wordt hem of haar met ingang van de eerste dag van de volgende maand
bij koninklijk besluit eervol ontslag verleend.
Translated by me:
When the chair of a water board reaches the age of 70, they will be honourably discharged by Royal Decree in the following month.
Unfortunately, I don't know what specific law it is that stipulates this, but I imagine it is the same law that applies to mayors. I did search the 'Waterschapsbesluit' ('law on the water boards') but could not find the relevant passage.
Example
According to this document by the Hunze en Aa's Waterschap, on the first of August 2017 their dijkgraaf's appointment had to be ended because he turned 70 in the previous month. The word 'rechtswege' translates as 'legally' meaning the ending of the appointment was because of some law.
The relevant passage from the aforementioned document (in Dutch):
Omdat de dijkgraaf op 8 juli 2017 de leeftijd van zeventig jaar bereikt eindigt zijn benoeming van
rechtswege op 1 augustus 2017. Dit ontslag wordt bij Koninklijk Besluit verleend.
Roughly translated by me:
Since the dijkgraaf turns 70 on July 8, 2017, by law, his appointment ends on August 1, 2017. This termination is granted by Royal Decree.
