I don't know that there would be an age limit, but the main issue is whether the passenger can look after themselves independently, go to the lavatory without assistance and be medically stable for the duration of the flight.
Cabin crew can't provide personal care such as feeding or helping to the lavatory. Wheelchair assistance can be pre-arranged through the terminal and to and from the aircraft. If transit is required for flight connection (as with the planned route), then assistance can be pre-booked if needed.
If a family member or carer is travelling with them, and can provide assistance, that may help the less independent to travel.
As a doctor, I usually advise those asking if they can make a certain trip, to see the their health is stable at the time of travel, and they can manage to sit in an armchair for the equivalent of the duration of the flight!