Apart from Christ it is impossible to get to heaven. Those who have not reached an age of understanding or have no capacity to understand (mentally handicapped people) not included since they are covered under His blood. There is no specific age either as I believe that differs for each person. The only thing I can say about those who have never heard is what the Bible says.
Romans 1:20
For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
I believe that if someone really wants to know who God is based on the evidence of creation, then God will send someone to tell them. That is how God works. If they reject that then there fate is sealed.
For you and I, we have heard the truth so it really does not matter about some tribe in the middle of nowhere. If you reject the truth, then you will be judged. That is what needs to be your concern.
My Tag: 2 Timothy 3:1
Jesus Christ is the ONLY way to Heaven! (John14:6)
Every second 2 people die . . . What if this is your second?