>>149829441
The writers probably just haven't thought much about her age.
We also don't actually know for sure HW's age, I don't think it's ever been said.
My headcanon is that she isn't actually much older than Finn, based on the fact that her Vampireworld counterpart is a regular human, and her Farmworld counterpart was likely a human similarly aged to Finn, since they were a couple and had kids.