Gorilla trekking in Uganda (bwindi impenetrable Natural forests or Mugahinga forests) or Rwanda (Volcanoes National Park) is allowed from 15 years of age, however some instances where a child below 15 years may trek gorillas if is a special arrangement that has been organized between the parents of the child the authorities, before tracking the gorillas, the national park authorities always ask everybody has to produce his/her original passport to ensure that every body is of the right age.
It is important to note that gorilla trekking is not an entirely safe process and these are wild animals and so behavior can change depending on the different mood and surrounding. It is believed that may be a child 15 and above can help themselves just in case anything happens and they are mature enough to follow the entire gorilla tracking rules.
The Gorillas are emotional and usually fear and are scared by things like the flash light. They also charge depending on what they feel; sometimes they are overwhelmed and feel challenged so they want to fight back in self-defense.
The forest is thick and has a lot of trails within and so it may be difficult for a child below 15 to cope with the map and may suddenly go off trails as they too playful.
Lastly, kids are naturally playful and might slow the pace of the person responsible for them which indirectly affects the rangers and the group at large despite the limitations on the amount of time you are supposed to spend in the forests. A kid who is at least 15 years of age will easily follow instructions and be less playful whilst on the trek.