Is it ok to get a subwoofer in an apartment or will it disturb neighbors?

Subwoofers can be set at any decibel level so you can figure it out how loud to make it.Cement is a pretty good buffer.

You'd be better-off using one of those headphones that doubles as a radio.  Then, you can listen to loud music without disturbing your neighbors.  

30 years ago I had  a ten wpc panasonic amfm w  rather large home built cabinets ( which I reconed and still use)

 it shook the walls of the indian guy who lived the next flat up, who did complain a couple times

.todays systems are 50 times  more powerful,,get a medium powered set and a couple pair of headphones

As long as its a small one shouldn't be too bad. Get a 6 inch or 8 inch subwoofer place it on an isolation platform and you should be able to get some bass and also not disturb the neighbors.

Subwoofers can be set at any decibel level so you can figure it out how loud to make it.Cement is a pretty good buffer.

Depends on the volume, the maturity level of the owner of the Subwoofer and what measures they take to ensure it doesn't bother the neighbors, like uncoupling it from the floor with an isolation pad to reduce transient vibrations/bass travel. 

If you're going to get a subwoofer for normal uses like watching home theater at an acceptable volume or listening to music at an acceptable volume I wouldn't worry too much.  but if you plan on getting a subwoofer and throwing 400 watts behind the subwoofer and playing boom boom boom music I can absolutely tell you that very soon all your neighbors will file against you and you'll be out of there

Keep the volume WAY down.