Good God, I’m an actual full-blown adult. Christ.
Sympathy! It doesn't really get any better, but you do eventually get more or less used to it. At any rate, that's my experience from about 20 years further into it.
And no, there isn't really a point where you have your life together, it's just a continuing process of trying to make it up as you go along.
-- lowering expectations today, for a less disillusioned tomorrow --
Oh, the comic discussion. Yeah, it was a kind of a slow batch for major plot points, but they can't all be high drama and there was some decent character development (as you said). You did ask for other perspectives on the age difference thing - I think that, like most human questions, you can't really make a blanket statement. Something that's merely uncommon in general can't be ruled out in a specific case. There are a decent number of unusually mature 18-year-olds out there and Ellen may be one. (And Steve may be less mature than the average 24-year-old, too.)
Thinking about it, I'm not certain that emotional maturity is even a big factor in connecting with someone - to my mind it comes more into play when you start talking expectations and commitment. Which obviously is important if you're aiming for a long-term relationship, but it's not clear to me that that's what either of them is pursuing at the moment. And even if they are, if they're both looking for the same thing, I don't see it as much of a barrier. For example, my in-laws were married at the ages of 19 and 28, and are still going strong over 50 years later. And yes, the world was a bit different back then - but people, I think, are still pretty much the same.
Of course, even assuming that the relationship is doomed to flounder because of this - well, it wouldn't be the only doomed relationship to feature in QC