Most emtbs have a low stand over and if the bike also has a dropper post then seat to pedal distance is far less critical. I only ever have the saddle fully extended if I have to ride a tarmac section, otherwise offroad I run the dropper partially slammed. That lowers centre of gravity and makes control on the trail easier. On an emtb the traditional correct leg extension for pedalling is far less critical because of motor assist......and most emtbs have shorter cranks than road bikes.
So the more critical frame dimension when choosing between medium and large is reach. ....or effective top tube (distance between seat post and stem ) but not all brands provide that detail.
Yes, you will only find out which size is best by going to a bike shop and doing some test rides but perhaps the biggest factor will be what type of riding you want to do. If you are going to ride mellow trails remaining largely seated size large may be the better option, if you are more adventurous/ active on the bike a medium would probably be more lively. Ps...age is not a factor! I'm 75 and 6ft and ride a large......and ride gravity runs with jumps and drops etc.