"18. Yes. Essentially all societies have created some form of elite classes with more rights or privileges than others. The sources for this power have varied immensely but you can notice them if you observe it well."

Answer to the assumption:
"the 'elites' -which is comprised of only adults-"

The assumption in question... is yes-and-no.

One conspicuous sign of a society having a true elite is wealthy child burials. Which means that elite status is openly recognized as hereditary and shared by children of elites - but of course not all children! In contrast to societies where the elites give their children a head start in life but pretend that the elite status is achieved by merit as adults.

But while children who are openly recognized as hereditary parts of elite share a bunch of privileges of their parents, they do not share the full agency of elite adults. In that sense, they are still not full part of "elite".