Breakdown of Han spøger ofte med sin alder.
han
he
sin
his/her/its own (common)
ofte
often
med
with; by (bus, train) / of, to (the meaning or point of something)
Questions & Answers about Han spøger ofte med sin alder.
Wait, doesn't spøger mean "asks"?
You are thinking of spørger (with an "r" in the middle). The verb at spørge means "to ask". The verb used here is at spøge, which means "to joke" (or "to haunt"). They look and sound very similar, so this is a classic trap!
Why use med when the English translation says "about"?
Prepositions rarely translate one-to-one. In Danish, the fixed phrase is at spøge med noget (literally "to joke with something"). If you tried to say spøge om, it would sound unnatural to a Danish speaker.
Would it be wrong to say hans alder instead of sin alder?
Yes, unless you mean a different man! Sin is a reflexive pronoun showing that the age belongs to the subject of the sentence (Han). If you say hans alder, it means he is joking about some other man's age.
Why does ofte come after the verb?
In a normal Danish main clause, central adverbs like ofte (often), aldrig (never), or ikke (not) must be placed right after the first verb. English usually puts "often" before the verb ("He often jokes"), but Danish requires Han spøger ofte.