The original was from MSN posted by a db0 user. Hexbear and ML users argued it's fake and asked for proofs. OP got some as a non-speaker, then I researched it myself and wanted to add what I've found, but the post got deleted before I replied.
So here is an english article about that: https://khpg.org/en/1608813775
And my comment with russian language proofs.
Local news site based in his hometown about that: http://www.uezdny-gorod.ru/novosti/litseist-iz-liven-prigovoryen-voennym-sudom-k-pyati-godam-kolonii-za-uchastie-v-terroristicheskoy-or.html Federal registry information is at the bottom, so it's legal status with russian gov is verifiable.
I'd add an article from one of the oldest NGO Memorial to add to what you posted: https://memopzk.org/figurant/turbin-arsenij/ They are specialized on repressions too, but originally on rehabilitating soviet political prisoners and collecting historical data in archives. Not now, though.
I'm a native speaker and after googling around I conclude that sources that are loyal to power doesn't mention Арсений Турбин at all and many but one post in VK about his sentence are banned via the order of Генпрокуратура originated in the first day of war. More about this order that's used against maaany resources: https://roskomsvoboda.org/en/post/ban-1500-ssylok/
As a bonus: I think I've found his VK page. Name fits, city fits, soccer interests fit, age is faked by adding years to avoid age restrictions, political position is not democratic but rather pro-Wagner and anti-Putin, more like an 'angry patriots' crowd. Not a direct link due to good taste rules, but he is googlable by his name I put in bold and the end of his handle is numbers. This account is new but it's created and started posting before the war. These articles mention another status in his bio, but as it wasn't posted, I can't tell if someone changed it - yet, it also tracks with Prigo worship.
My opinion #1: The boy is real, he is charged and his process is silenced in federal news, everything posted kinda tracks. We can't verify his connection to the armed russian fighters on ukrainian side, but he doesn't like Putin and was fanboying after Prigo's macho vibes, so he could try to contact them, and then dropped it? That part of the story is told by NGOs with a big local rep, so I trust them to fill that hole.
My opinion #2: OP has probably mistaken ML instance for another one and after the mods noticed it (and proofs started to appear) they deleted the post due to their bias alone.
I've not posted there yet, so if it doesn't fit, please tell me. Also, keep in mind that he is just a 15yo boy and he's hardly thinking for himself.