Gorilla tracking families are allocated to trekkers based on their age and physical fitness. One's physical abilities to hike and travel short distances determine which gorilla family to trek with. check this recommended 5-Days Bwindi gorilla trekking tour

For your information, Gorilla groups live in areas of different altitudes; some are at low altitudes, while others are at high altitudes. Therefore, the difference in altitude comes with a difference in the ease and toughness of the hikes. 

Travelers who are physically fit are given gorilla families in high-altitude areas, while those who cannot hike high are given those in low-altitude areas. Also, travelers are always reminded to disclose their fitness to their guides so as to reach the families they are capable of reaching.

Tourists who are totally unfit, sedan chairs have been provided, and travelers are carried by porters to see mountain gorillas and then carried back after an hour with the endangered species. see all the available tours here 

Age is yet another feature they consider in the allocation of gorilla families. In most cases, elderly people are given a chance to trek gorillas in low-altitude areas, while young one’s trek gorilla families in high-altitude areas.