You're good, no arguments starting here.
Maybe they've not changed lore-wise (considering no lore to give them in depth characterization was given to them in the first place), but personality wise? Absolutely.
I mean, sure, Miyamoto said that, but that was all the way back in 2012, and he seems to play no role when it comes to writing the Koopalings for anything nowadays.
The biggest reason I'd attribute to why it seems like some of them are older is because of the games Bowser Jr's Journey, Minion Quest, Paper Jam and Color Splash. These games are pretty much the only modern pieces of media that explore their characters in depth. From the way they speak and act, we get an idea of their age range. They all definitely seem to be at least a bit older than they were back in '88 (for example, compare Larry's dialogue in the smb3 manual to the way he speaks now. Before, it was pretty clear he was a very little kid, but now, he seems to be like a pre teen or young teen). Another reason why is because some people consider the order in which you fight them in smb3 is the order of ages, from youngest to oldest. Another reason is because of a small canon fact; Ludwig is the oldest, Larry is the youngest. According to a YouTuber who contacted Larry's current voice actor, they were told that the actor was instructed to give an impression of a 13 year old boy during auditions. If that is Larry's canon intended age, then there's some gaps in years between the siblings. How big? Who knows?
So, because of the unknown, some people interpret them as adults, others as children. This causes conflict, because obviously it would make most people uncomfortable to see someone say they're in love with a character that they believe are underage. And then the arguing ensues... Next thing you know, a lot of people become VERY annoying and turn it into a fandom wide problem by posting about it under the main hashtags, and just arguing with anyone who crosses their path in general that doesn't share their same ideas.
That's why I made this post; to remind everyone that the argument is dumb, a waste of time, and to know when to block and ignore people. It's the internet, no one can control how others think and act. You are entitled to your own opinions and boundaries, but it is not an excuse to start shit. Neither side has concrete evidence to prove that they're all adults or kids, so therefore no one has the room to be talking the way they have been.
Sorry this is a very long response, but I hope it explains some things.