When I was 18 got in some misdemeanor trouble. My question is will misdemeanors show up on a FBI criminal check and for how long? I'm now 38 years old and have to get fingerprinted for a job.

Well, it depends on what your application asked for. If you had ever been convicted of a crime or have you ever been convicted of a felony.
Either way, I would imagine it will show up.

It is a most likely, 20 years ago, many minor misdemeanors were not fingerprinted and arrest records done, it was not unusual to be thrown in a cell for 23 hours and released after been taught a lesson.

But if you were fingerprinted and appeared in court, yes all misdemanors will show up on a criminal background check.
*** now this is the check the police run, remember all for the private search companies don't have access to all police records, so some searches are better than others. But it can. But to be honest most minor crimes 20 years ago don't effect much

If you were convicted then yes, it'll show up. The exact nature of the offense will determine whether it'll impact your eligibility for the job.