i honestly think the writers really shot themselves in the foot (as they like to do) with the miraculous timer not existing specifically for adults instead of that just being a skill that has to be trained. The training montage in the beginning of Revelator was cool (in theory bc why is this only a two minute scene) but the fandom‘s main concern seemed to be about how fucked up it actually is bc this is basically forcing kids to grow up too soon. And I see where they‘re coming from, but also the lore for the timer is so unclear overall.
I fully agree. Calling the timer upgrade an "adult" power then giving it to the teen heroes is just weird. Just say that it has to do with experience or emotional maturity or something like that (not that Gabriel was ever emotionally mature, but let's ignore that issue for now). Everyone was anticipating this upgrade being a thing that happened when the characters turned 16 or 18. Having it just randomly show up while most of them are still 14 (15?) raises all kinds of questions and makes people give the upgrade undo weight.
I had another ask about this recently and in that one I talked about how lackluster this upgrade was. The characters don't actually mature, they just kind of will themselves into "adulthood" which is one of my main issues with the complaint you mentioned. A complaint that I have also seen a few times from different sources which is why I feel comfortable addressing it:
the fandom‘s main concern seemed to be about how fucked up it actually is bc this is basically forcing kids to grow up too soon.
It's hard to view the characters' "adult" status as "forcing kids to grow up too soon" when this upgrade is the least serious thing that has happened to most of these characters. Forcing Marinette to try to navigate the complex mess that Gabriel dumped on her lap is "forcing kids to grow up too soon." Luka being banished to Brazil because a terrorist wanted to capture him and force him to betray his friends is "forcing kids to grow up too soon." Adrien having to navigate being an orphan with no forewarning that his father was dying is "forcing kids to grow up too soon." The list goes on.
However, none of those things are why the characters are "adults" now. They're "adults" because of a rather generic pep talk. It's also worth noting that the scenes where the characters becomes "adults" are framed as moments of empowerment. That means that, narratively speaking, this is not supposed to be seen as a bad or concerning thing.
This show's messaging has always been wacky, but I'm pretty sure that this is less a commentary on how messed up these teenagers lives are and more the writers just wanting to show that the characters are growing up because we're six season in and growing up is not some great evil. It's just a normal part of life. That's why my criticism of this upgrade is not "upgrade bad" or "upgrade depressing" it's that this should have been part of a character arc where the characters actually grew up a little!
Also, complaining that the teen characters are "growing up too" soon feels too much in line with people who complain about shows like Miraculous "glorifying child soldiers." It's an action show aimed at young kids. The lead characters are going to be kids and teens because those are the kinds of characters young kids relate to. If you don't like seeing kids and teens being given responsibilities that they shouldn't have in the real world, then don't watch these kinds of shows. To enjoy them, you have to be willing to suspend your disbelief about the age issue and treat the responsibilities these characters have as fine in their world. Or, at least, not concerning because of their age. It's less "this is bad for a 14-year-old" and more "this is a terrible thing for anyone to deal with."
This is why you'll see me do things like treating Nathalie, Felix, Kagami, and Amelie as equally responsible for keeping Adrien in the dark in season five even though Felix and Kagami are "just teens". It's also why I don't really talk about Marinette being "just a 14-year-old girl." I get that defense, but I don't think it fits this genre because, if you're going to use that defense, then why is she responsible for anything? She's too young for everything she's doing! All the teen characters are, but is anyone arguing that they should all quit and be replaced by adults? I don't think so. If we put this stuff in adult hands then we would have a wildly different show.
I do think that the teen characters' age has validity in certain discussion - for example, I think it's perfectly reasonable for them to have teenage struggles because they are teenagers - but generally speaking, I'd avoid using age arguments when it comes to the characters' responsibilities and authority in shows like Miraculous. Characters in these shows are not supposed to be treated like normal teens by the audience. That just doesn't make for a good story.
Along similar lines, I don't like the "this character is only 14" or "it's only been a year in canon" arguments to defend things like the lack of character growth. That's just not a good defense for a fictional story like Miraculous. It's an action-adventure romcom, not literary fiction. It's not supposed to be hyper realistic. It's supposed to be a fun and engaging story. It doesn't matter how old the characters are or how long it's been in canon. What matters is that we're six seasons in and that season five saw the end of the show's first major story arc. By that point, it's perfectly reasonable to look for character growth and satisfying plot progression.