I played PS I in 1991/92 when I was 7 I think. But it was a long time before I played the others on emulation, so you can imagine how much I played PS I.
I'd heard about PS II and knew what the gist of it was but I never got around to playing it until after I'd played PS IV. But I knew PS IV took place 1000 years after PS II and PS III was something different altogether, so I wasn't too confused by it, though the difference between PS I and PS IV was astounding in comparison. The fact that the characters had more than a few lines of dialogue was brilliant. They seemed alive in comparison to PS I.
PS III: well same as Tanith, except I was older. But when I found out the whole story behind the development it became a bit clearer. And it took a bit of time before I got into it and played it properly. But after a while, given a chance, it actually won me over and I quite like it.

As for maturity, well PS I didn't do me any harm. I am amazed I was able to understand what was going on though, I remember seeing the intro for the first time and actually finding it a little sad. But the gameplay was a good change of pace from the glut of platformers and such out there at the time. I would probably say the same about the others too.
It's definitely earned it's place as my favourite game series for sure. Which is probably why I cringe a little when I hear the old favourite game argument come up. The way I see it, 4 great games all with their own ups and downs and different from the others but not a bad one among them. How often do you get a series of 4 games that are actually good and consistent? Be thankful for it.