Everything is relative. You might say "But murder is not allowed in any society!" but definitions of murder may vary. So in one society euthanasia might be allowed, but not in another. In one society abortion is murder, and allowed in another.
I agree, the suggested test for determining if something is absolutely moral would lead to that kind of off result.
Let me ask you this: we can test theories about how light works by making predictions about how light will behave in some circumstance and then by running an experiment and checking if the prediction was correct.
If instead I have a moral theory which suggests something is absolutely moral, what prediction can I make based on that theory?
You're so sure Ancient Greeks were categorically wrong, in their behavior towards individuals modern society would consider underage?
Hmmm, something tells me that their era was profoundly different in serious ways that aren't captured in recorded evidence that is available to us.
Before you even get to social interaction among peers, simply weather, disease, medicine, wild animals and poverty were all probably profound dangers to everyone across the face of the earth.
Nevermind literacy, and writing, just imagine how many normal human beings were completely feral, or mute, or inacapable of communicating verbally, for a wide range of reasons, including growing up in isolated wilderness and simply never learning organized speech, as part of a formal language.
Anyone who might help another person by sharing food and staying warm was probably of marginal pratical use, until the next period of hard times, either because of the random of marauders or nature taking its course.
I'm pretty sure healthy people who you could hold a conversation with were in short enough supply that once familiar, everyone made quick use of any luxuries available. No books or formal education, meant bootstrapping these things as new ideas which had no generational inertia, which means probably very nearly everything for most societies was very comfortably (or not comfortably at all) based on oral traditions.
Also people fucking died. Early. Lots of people's teeth were probably gone by 25. Blindness in an eye or both was probably kind of a little bit normal by 30 for many.
So, age was probably a different thing back then. In places where misery is coming from all directions, I'll allow for degrees of moral relativism. Especially for any period pre-dating the emergent modernity of ancient Rome. Any nomadic society that can't exactly distinguish diseases from curses and witchcraft, or even weather and plagues from punishing deities, kind of gets a hall pass.
