I retain both my points that 18-24 years don't merely act less mature, but in fact are not fully biologically developed yet. I agree with you that Dumbledore's recruitment strategy is dubious at best, but at the same time, there is no indication that there were all these willing adults ready to enlist in the Order. People trusted the Ministry and the Aurora to take care of the Death Eaters and Voldemort--the majority of wizarding adults seemed to trust the Ministry more than Dumbledore or Harry right up until the Ministry was effectively taken over by the Death Eaters. There are many indications of this--look at how Seamus reacts to Harry in book five because of what his mother has been saying about Dumbledore and Harry. I think that's meant to give the reader a pulse on public opinion at this time which was stacked against vigilantism. So I don't see any clear evidence in the books that adults in the first or second wizarding wars were largely willing to disobey the Ministry and fight. In fact, I see more evidence to the contrary. The wizarding population at large seems to face the Death Eaters and crises in general with deep, deep denial and avoidance. It's a reflection of the real world, after all--how many of us are really willing to act and stand up to injustice when it might lead us to harm, or even when it might just mildly inconvenience us by requiring us to get out of bed on a cold morning and attend a protest at the Capitol? Again, it is morally wrong to demand young people to so obviously risk their lives, but in the series, they seem to be some of the only people really willing to do so. Of course, there are adults like Moody and Mcgonagall, but how many would the Order really be left with if it was just them? They needed the young recruits, because individuals like Moody and Mcgonagall who were willing to step up and saw the weakness of the Ministry were few and far between.
I think you can have someone's best interests at heart while perhaps being confused about what they are or messing things up along the way. Don't we all do that, even if the stakes are rarely so high?
How can we not be talking about the literary importance of his decisions? These are books, after all, with themes and symbols and an author who has certain plot and character trajectories that she wants to see met. I don't think it makes any sense to look at a character in isolation of other literary devices, since characters are in essence literary devices as well. You're not seeing the character in context, then, so how can you make a reasonable evaluation of the character? For example, the main theme of Harry Potter is love, but love is not a uniform thing. People can love and be loved in different ways--Sirius gives Harry the best estimation of the love if family in the absence of Harry's real family. He's the closest living person to Harry's father. So his love is flawed in this way, too, because he's only a representation of James, likely struggling himself with treating Harry in a way that James would and not even truly knowing what James would be like as the father of an adolescent, since he never was the father of an adolescent. This is a messy, complicated kind of love that shouldn't even have to be, because in an ideal world, Harry would have a real father. It's compounded by the fact that Sirius is certainly traumatized and not fully well in the long shot--but, nonetheless, it is love. It's connected to the overarching theme of love, and that's why you can't isolate the literary significance. Further, you can't dismiss literary significance, because Sirius is not a real person, he's not even a main character--he's there to advance Harry's narrative, which is one of finding independence and how to trust yourself above all others. So to say Sirius should have done X, Y, or Z if those things may have kept Harry from becoming the person he is at the end of Deathly Hallows is futile.
Ah, parental roles--need parental figures be perfect? Or might Sirius not be an imperfect parental figure?