Yes it is as always a judgment call. Somebody using a hearing aid would not in my opinion warrant automatic exclusion from being seated next to an emergency exit. If the person met the general criteria and could hear reasonably well with an aid that would be fine. As has already been said somebody with corrective lenses would be considered suitable.
The persons seated at an exit should be prepared and able to assist in an evacuation should that be necessary. There is no guarantee that anybody sat in an exit seat would actually be able to perform if required, so it is only ever going to be a case of best judgment at a given time. A crewmember will be charged with the control of any given exit.
On the subject of glasses, most Captains over 45 will have developed some degree of presbyopia and require corrective lenses. They will certainly be qualified to sit at an exit, so that point becomes a bit moot.