Two abbreviations you may often find in older marriage records are J.M. and J.D.
“J.M.” stands for Jonge Man, literally: young man.
“J.D.” stands for Jonge Dochter, literally: young daughter.
They refer to the marital status of the person, indicating they are single and this is their first marriage. The word jong/young has no relation to their age.
Sometimes, you find J.G. [Jong gezel, approximately: young journeyman or companion] instead of J.M., but the meaning is the same: a single man.
In court records, you may sometimes find the letters J.D. behind a man’s name. In that case, it indicates the man had a law degree.