Check here how the boarding process works in each age group:

From 0 to 5 years:

You must present a birth certificate;
Must be accompanied by parents or legal guardians who present official documents proving the bond;
The child up to that age is entitled to the benefit of gratuity, as long as he does not occupy an armchair;
If the child is going to travel with the benefit of gratuity, it will be necessary for the parent to collect a free boarding pass in advance at the company's counter.

From 6 to 15 years:

Minors of this age pay the full price of a ticket;
If the minor is going to travel with his parents or legal guardians, he will need to present his identity card or original birth certificate;
The minor must travel accompanied by relatives up to the 3rd degree (mother, father, siblings, grandparents, uncles, great-grandparents, nephews over 18 years of age or legal guardian). It is necessary to prove the kinship by the documents and if the document is not presented, the boarding will be prohibited;
If there is no relationship between the minor and his/her companion, a written authorization signed by the mother or father with a notarized signature will be required;
If, exceptionally, the minor travels unaccompanied by parents or legal guardians, he/she must present an identity card or original birth certificate together with judicial authorization.

It is worth emphasizing that these are the general rules for interstate (between different states) and national travel.

If your trip is intercity (between cities in the same state), we recommend that you consult the responsible bus company.

{Tradução Google Translate}