There are no upper age limits at most African safari camps. Some activities, such as canoeing or hiking require a certain level of fitness from the participants, but provided your parents meet these requirements, there are usually no restrictions. There may be a maximum limit for endurance sports or activities such as mountain climbing. Best to check with us and advise us about any age related issues while discussing the trip.
From a purely pragmatic perspective, there is no reason for your parents or other senior citizens not to be able to handle a safari, but they may want shorter days in the bush or other amenities that may require booking a private vehicle for the convenience of all concerned.