Oh dear, this is like several threads rolled into one. Let me try and break it down:
Should people start a novel as soon as they like or write shorter works until they hone their writing skills?
I think everybody is different. There's no set formula for how to make a good writer, because like any other type of art, "skill" is entirely subjective. Of course, there's a technical aspect that one does need to master, but other than that, there's really no saying. A writer does need to write before creating a decent novel, no doubt, but it doesn't necessarily have to be shorter works.
I would like to write longer work at some point, but would it be wiser to concentrate on short stories until my style becomes good enough to be publishable? (That or should novel writing be purely therapeutic for enjoyment?)
Obviously, every writer wants to be published, and we all have daydreamed once or twice about a huge Big Six publishing deal. But that's obviously not what it's about. If you worry about being publishable before even writing the thing, than your mind will not be where it needs to be to create good art. So, to answer your question, yes, I've always been of the belief that one needs to write for recreation and enjoyment before he/she can really worry about publishing.
Along with that, how does one decide whether his or her idea is good enough to expand into a novel rather than a short story? Or are those two mediums even comparable?
This question is a little confusing. Are you saying that ideas for novels must come from short stories? If so, no, absolutely not. As you suggested, novels and short stories are two totally different forms (well, maybe not totally different, but certainly not very comparable in composition or style). Or are you saying that the way to get better at writing is to write short stories? To which my answer would also be no, for the same reasons I listed above. As far as gauging your talent as a writer goes, I think the best way to go about it is to get feedback from people whom you trust. I've found that I'm a terrible judge of my own work. Many pieces I've rather liked have been ripped apart by betas and others, and some pieces that I didn't like so much were applauded.
Whether an idea is good enough for hundreds of pages?
I don't think those two things correlate. A good idea can be one page or ten pages or one thousand pages. A novel is multiple good ideas, good plots and subplots, stitched together.
When did you decide to write a novel?
I'd started writing many novels when I was very young, and finished none of them. I got to about 25 or 50 or even 100 pages every time and hit a dead end or became disenchanted with the story (which is something any novelist must be prepared for by the way. This still happens to me.) When I was when I was 17 years old, I finally completed the first draft of a 90,000 word manuscript. I think I'd just been going through some really tough, personal things at that period of time and I couldn't properly articulate it through poetry or through short stories the way I had been. I also think it was at a time where I felt I had read enough literature to have a proper understanding of how novels work, if only in the most basic of ways. I've only attempted one other novel besides that one, which I'm working on now, and there's not really much of a connection between the two, topically, stylistically, or otherwise. I think you just have to find a topic or a theme that interests you, and also one that you are willing to commit a large amount of your time to.
Anyways, I hope some of this made sense and was helpful!