Indigenous in Bolivia

One reviewer of my 2nd edition U.S. and Latin American Relations textbook referenced the 2012 Bolivian census to get a more accurate percentage of Bolivians who are of Aymara or Quechua descent. Reading it, though, I realized it's not terribly useful because it only asks respondents over the age of 15 how they self-identify. That leaves out just around 1/3 of the population.

I understand we might say that children 15 and younger are not in a position to self-identify, but without them how do we make definitive conclusions about the entire population, especially one so young?

Really, the more you look, the more obviously difficult it is to get a firm grip on the percentage of the population that is "indigenous." In 2001, for example, some 20% of the Bolivian population self-identified as indigenous despite not having any "recorded ethnolinguistic marker" that would suggest they likely would be.