what is Neil Young's song Sugar Mountain really about?

Religious incest cults

google is your friend
friend

Sugar shack, sugar house, sugar hill, etc., typically sugar in these contexts has a sexual connotation, e.g. whore house.

Drugs most likely.

About going back to his lost youth, I think he was only 19 when he wrote it.

This one didn't have that kind of meaning, pretty sure it didn't.

In which case it's unlikely he knew about the slang definition, so the wikipedia explanation is probably correct, but the lyrics also lend themselves to something like a critique of secret societies although again, he would have been too young to know about that.

Yeah at 21 years of age you're entering geezer territory, it is traumatic ;-)

unless he was a member of a religous incest cult

A woman's pubic mound

Or

CO FUCKING CAINE!

because at 21 you automatically become monk-like

JONI MITCHELL SAID IT WAS ABOUT HIS NO LONGER BEING ALLOWED TO PLAY AT AN UNDER 21 CLUB IN CANADA ONCE HE BECAME AN ADULT. SHE KNEW HIM, MAYBE ITS THE TRUTH.

I can imagine a bunch of men, society of the hidden hand, keeping a close eye on up and coming artists looking for one that could become a viable investment opportunity and then they start grooming him with handlers, etc.

Big Brother and The Holding Company

She could be MI6

YOU MEAN FULLY AUTO?

The song could interpreted as a loss of innocence. Now he knows the ugly truth behind the happy illusion.

He's been exiled from paradise after eating from the tree of knowledge.

you can't be 20 on sugar mountain

Google is nobody's friend.

emm eye six

not

emm sixteen

I doubt it represents an allegory about exile from paradise because otherwise he would have have had an existential crisis and ended up like Shelly Duvall

It's about Willie Nelson being tricked into signing the 1040 form the very first time. Willie wanted to be a free man and owe no allegiance to man.