These examples come from both the end of S6 and the beginning/middle of S7.
- Buffy ends the relationship with Spike, builds a better bond with Dawn (yes, it can be argued that this tapers off mid-S7, but it is true that Buffy trusts her more and gives her more responsibility), APOLOGIZES to Dawn, etc.
Dawn's fine, she's a high school kid. However, Buffy ends the relationship with a known arms dealer, presumably illegal weapons since Riley was after him. And instead of making sure that he's brought to justice so that he can't actually just try it again, she simply walks away. That's not maturity.
Nor his her incredibly flawed argument with Giles about why Spike won't fall victim to the trigger again. She's provably wrong, even though nobody has the smarts to point it out.
There's also her general tactical incompetence, but that's more directly season 7.
- Willow goes to England to accept her punishment, is serious about her rehab, trains herself to control her magic, comes back to Sunnydale even though she is afraid to face her friends..
And consequently uses magic to force thousands of girls to become slayers. Instead of, say, calling the military for help, which they were willing to do quite readily to replace Spike's chip. That's not what I call learning about responsibility with magic.
- Dawn perhaps has the biggest transformation..from a whiny, immature klepto to the young woman she is in S7. "Potential" showcases this nicely.
Perhaps, but if it's supposed to be a theme, if one of the only real examples is Dawn, then it's not that great of a theme.
- Anya regresses a little out of grief, but by the middle of S7 is more mature than she's ever been, she's independent and learning how to be just Anya.
A little? That's what the wanton slaughter of, okay douchebags, is? Or are you talking about her horniness? Or her general lack of tact to the point of interrupting important discussions to get some snark in?
- Xander comes to terms with his place in the unit...he is the observer, because it's all he can be, but he finally embraces this. Also he realizes that his greatest power is his love (which he uses to stop Willow from destroying the world.) He learns a lot and grows a lot.
Which is why I didn't criticize Xander per se.
I know plenty of adults who make stupid decisions. People make mistakes all the time. People, even adults, have errors of judgment all the time. That doesn't mean that no one is an adult unless they are perfect.
It's that most of these characters make stupid decisions that cost people their lives that's the issue and why they're not very grown up. It's one thing to screw up, but not when it ends with a bunch of girls getting their throats torn out because nobody figures out that an FAE would make it a lot easier to face the turok-han than a pointy stick.