The wax seal is usually the cause, but it's possible (Depending on the plumbing configuration) it's also possible that your vent stack is not drawing well, clogged and the sewer gas can leak back through the vanity which also might be in need of some attention as well.
Usually in an old house which was in order and developed this sort of problem at some stage, its due to wear and tear,
that's why the most likely weakest link of failure is the wax seal.
I would also recommend while you make this repair to check the level of the floor in relation to the drain pipe on the ground, often the reason this wax seal fails is because floor under it is either too LOW, where the seal doesn't make perfect seal,
or too high causing the wax to get squeezed out and open craves to leak through,
now days they have multiple solutions to fix this by picking (spend a few bucks more) alternate off the shelf seals to overcome it.
I like using the wax seal coupled with rubber trunk which ensures waste gets routed into the pipe.
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