I've been thinking quite a lot lately about the phrase "life imitating art".
If you're talking about film, the two never really imitate one another but they can often be similar reflections of the other...
There can be no art without life and art is the expression of life, so the two are very tightly linked to the point where in my mind, there's very little distinction between the two... Virtually everything that we create that is the product of an expression of our emotions can really be considered art. Regardless of what religion you subscribe to, each one of us is the ultimate expression of human emotion, and could be considered among the greatest works of art of all, especially in the eyes of our parents... :). The degree to which you consider any creative product in terms of its aesthetic beauty and emotional expression is obviously what elevates it to the level of art as we think about it in the popular context.
The point I'm driving at is that when you make a film, it's the expression of something - typically based in an emotion that you've experienced or dealt with at some point in your life. The degree to which a filmmaker is able to connect with and express these emotions is in my mind what differentiates a good film from a bad film, or a good film from a really great film..
It is not just the story. It is not just the characters. It is not just the way the dialog is crafted. It has so much more to do with the connection that the writer or filmmaker has to the emotions or experiences that are being expressed in the film. Plot and overall story concept are of course extremely important, but they are nothing without good emotional connection to the characters. If the emotional connections are there, the other elements should fall into place quite naturally.
I feel that this is why the advice of a lot of veteran filmmakers and/or writers is to "Write (or stick with) what you know". I've interpreted this not just to mean stick simply to topics that you're familiar with, but that you should at the very least make sure that you do the best that you can to connect with the emotional journey of your characters. Perhaps one of the best examples of this is the master, Steven Spielberg. Regardless of the subject matter, he manages to connect with the audience. He does this by giving his characters real, relatable emotional struggles, goals and motivations. The situations he puts his characters in changes, but what doesn't is how he allows us to connect with them and almost become friends with them during the course of their cinematic (and emotional) journey...
So ultimately, I feel that the more mature you are with regard to life experience and your ability to be introspective and connect with human emotion, the more mature and successful your films will be. Notice I say "life experience" - I don't say age because age isn't always the best indicator of life experience. Spielberg is a great example again because he was all of twenty-six when he started filming on Jaws. However, in these twenty-six years he'd experienced tremendous upheaval and change in his life which obviously had a profound effect on him.
For myself, I can honestly say that in my forty-two years, there has been no shortage of life experience, upheaval and emotional struggle. Whether or not I'll find a way to truly tap into it all in a meaningful way as a part of my filmmaking process remains to be seen, but I view this as the central foundational element in the production of a successful film (with a "successful film" being defined as one that has the ability to connect broadly with an audience on some core emotional level).