I haven't read this in a few years, I stopped around the time Mo was accused of sexual ass...

adults

I think it depends. Like one chapter it’s them in the present (adults) and the next one a flashback (highschool). But the latest chapters so far are about them in the present

It jumps from HS to adults and in between sometimes xd.

Most of it is Middle School. Future chapters appear every-so-often, in holiday specials (Christmas, Chinese New Year, etc), but the main storyline of how they even get that far is still a mistery.
For some reason though, the last 7 chapters or so have been set in the future without any clear reason. This is the first time this has happened, however the main storyline is still set in Middle School, as we have yet to even see them graduate.

I genuinely feel that old xian just wants to abandon her original storyline lol. Main story progresses way too slowly and she is doing so many flash forwards. She has been making the chapters a bit spicier and I don't think she wants to do that if they're minors

I think she's more interested in Mo and He atm, and since their plotline goes on kind separately from the "main" storyline of Jian Yi's kidnapping and etc, she doesn't need to care much about their high-school days and what-not in this way

Bc honestly she always drew them being kind of "naughty" at times, so their ages wouldn't matter much in this sense. I don't think she can draw (and publish, that is) NSFW so I assume this isn't a line she wants to cross

I don't think age matters, because she does a lot of "hot" or pervy situations/innuendos even with them in High School, He Tian sent pictures of his D to Mo and when Mo was cleaning him, he took Mo's hand and moved it towards his D, the other couple has less moments like that but is more because they are far more "silly" and have another vibe, but I'm sure they had them too.

If them as adults are more open about it is because they are already together at that point, storywise, the couples are more comfortable with each other and their sexuality not because of age. (In my opinion)

Old Xian can't draw NSFW or something spicier than what we are getting because is illegal in China right?

I wouldn't say it is *illegal*, per se, but censorship laws exist that are they're mainly used to take that kind of stuff down if it does appear.