You haven't spent much time around young people recently if you don't think the vast majority of 18-21 year olds are still essentially children. Speaking as one, here. So I don't entirely buy into the argument that there would be scores of people in their early 20s who would be significantly more emotionally mature than the Marauders--or, at the very least, I don't know where these droves of level-headed 18-21 year olds are in the real world. The brain is actually still developing at this point, scientists now have discovered that the human brain isn't fully developed until the age of 25--so half the people in question died before that point, the other concluded his development in a prison guarded by depression-monsters.
On one hand, the real issue is that Dumbledore would recruit people so young--on the other, the Order is a vigilante, extra-legal organization, and if you look at real-world parallels, they are often manned by young people who are more ideologically fervent. The natural, more experienced pool would be experienced Aurors, but it seems the majority of these did not want to join a vigilante group, and with good reason. They are law enforcement officers, the Order essentially exists because Dumbledore doesn't think they're doing a good enough job, so why would they join?
I do think you're stretching it quite far to say Sirius didn't love Harry at all. Like virtually all Potter characters, Sirius is complex and deeply flawed, but complex and flawed individuals can love others. We can be loved by people who make mistakes, even those who make mistakes that have negative consequences for us. Not everyone is perfect, and not all love is perfect. If Sirius had done too good of a job looking out for Harry, it would have detracted from Harry's character arc--the whole idea of the series is that Harry needed to defeat Voldemort on his own, he needed to surpass his perceived mentors and find his own inner strength.