Balance:

To be able to have a good experience you should be able to step up one or two steps on a ladder and have some balance ability in that you might have to lean forward and stick a leg out behind you for balance while standing on one foot.  Of course you have the height of the ladder in front of you to hold onto, and it's really not hard at all for the majority of people.  Also, many objects are low and don't require the ladder except for the young or very short, and some tall people may not need the ladder except for the more overhead objects.

Vision:

The telescope is an awesome eyeglass and can correct for any level of vision correction needed except for astigmatism.  People with minimal astigmatism get the best experience looking without their glasses, while those with significant astigmatism will find using their glasses give the best views.  The eyepieces I use have eye relief appropriate for eyeglass wearers.  Contact wearers (myself among them) will have no difficulty.

Age:

The very young and the very old may be challenged.  In particular we want children to have the best experience they can have.  Their eyes allow them to see detail us adults can no longer see.  I'll never forget the first child that asked me 'what's that bright thing in the middle of Ring Nebula'.  For adults, the central star in Ring Nebula is invisible except in time lapse photos.  The youngest children however are challenged by the concept of looking into the eyepiece.  I recommend before bringing a young child to a star party that you teach them to look into an eyepiece by using one eye to a View Master toy, periscope toy, or microscope.