Belgian citizens are automatically registered on the electoral rolls when reaching the age of 18 and are subject to compulsory voting under Article 62 of the Belgian Constitution (Legislationline, “The Belgian Constitution”). Universal suffrage rights date to 1948, when women were granted the right to vote. The minimum age to vote was lowered to 18 years in 1981. (The Belgian Senate, "Parliamentary Election")

References:

Legislation Online, "The Belgian Constitution": https://legislationline.org/sites/default/files/documents/98/BELG_Constitution.pdf

The Belgian Senate, "Parliamentary Election": https://www.senate.be/english/LegElectionsEN.html