Is this more of an urban legend/truism or is there actually science to back it? As someone who’s 24 I find the idea I’m not truly an adult laughable and a little bit offensive.
Research seems to support it. But you probably won’t understand this for another couple of years or so. 
For most of human history, you’d probably have been married and popped out a couple of kids by 25. So the fact that one’s brain is still developing up to that point doesn’t necessarily mean “hey, now you’re a full adult, you can do full adult things, yay”. It might mean “Eh, you’ve probably done your evolutionary duty by now … time to trigger the gradual die-off process.”
Just in case anyone was looking for a cheerful thought to brighten their day 
Unless you are an idiot, you never stop learning, so your brain never stops developing until you die.
Exactly. By the way the link says “at least mid-20s”, so I think the idea you’re “done” developing at 25 is questionable.
Maturation definitely not till late 20’s.
As a victim of my own bad decisions in my early 20’s, I whole heartedly believe this.
And you don’t think this might be evidence that your brain is still developing?
/sarcasm
The way I’ve understood it, younger brains are geared toward more impulsive behavior, which is great because it leads to unplanned events such as the creation of children. After a time, it’s advantageous to slow down just a bit and think about keeping your progeny alive, and that takes a different mindset. Rather than count on your figuring this out all by yourself, your brain gives you a gentle nudge in the right direction.
That happens around the 25 year mark, give or take. Think of it an an event, a final state change for your noggin.
Still I highly doubt a 26 year old has more in common mentally with a senior than with a 19 year old or something.
I’ve had several intimate conversations with late 20somethings that go kind of like this:
“So…I feel like, I don’t know…like some sort of haze just lifted.”
“Yeah?”
“Yeah! Like, I can just think better. I dunno. It’s weird and hard to describe.”
“Yeah. And you were about 25, 26, right?”
“YES!”
Anecdotal, for sure. But I felt it. It was like a veil I didn’t know was there slipped off. It’s not hugely dramatic, and no one ever really talks about it, but yeah, it’s real. And it’s why we don’t have a whole lot of faith in teenagers.
Coming up on 32. I couldn’t detect much of a difference in my 20s.
I would say the most difference in thinking was between 18-20 (move out of the house, start being responsible for self).
Where was this ever suggested?
From your OP and subsequent posts, it appears you are confusing the underlying physical hardware of the brain with the software that runs on it. The brain and the rest of the body undergo various physical changes throughout life. These are not related to the legal notions of physical or mental maturity or more broadly to cultural notions of maturity.
In many ways, a 26 year old American will have more in common mentally with a 50 year American than a 26 year old San tribesman. That has little to do with the physical hardware of the brain and everything to do with knowledge and culture.
More anecdote coming:
It has been my observation that people change a lot between 20 and 25, moreso than in any other 5-year life period. I can’t say that it’s because of neuroscience and not life circumstances, but I’ve seen it happen in person after person.
It’s almost as striking as the change in children when they’re around 5-6 years old. They stop being little savages and turn into little people. I’ve always attributed that change to kindergarten, but I could be wrong.
Your friends must have been pretty late in development. I felt a change like that when I was maybe 21-22 or so. 25 or 26 might be the very latest it happens, but I think your brain continues to develop until you die. Or until you start deteriorating LOL. 
I know lots of people in the late 20’s and 30’s that are immature or act very teen like.
I’m sure people taking sociology and psychology can explain why that is the case.
I also have knowen teens that are more mature than some adults.
And some of us were just born old… ::: sigh ::::