What everyone else said about simply writing is true, but Ill answer your question as honestly as possible.
The average working writer makes $4000/year or so, according to one study. It really isnt the road to wealth. You could become good and get lucky. But its rare.I can literally name the 30 writers of fiction that make over 1 million dollar per year. (Most of them write romance). In a generation, who knows--you might be one of them.
First you need to write at least a million words, study the craft, and learn the basics of the business. Write your novels., Write a few short stories and send them to the fantasy magazines (this is also a good way to know how youre doing--total form rejections mean your writing is not yet good). VERY lucky people might start making a living as writers after only 8 years of hard work. It took me over 25 years. Most writers never get there.
A million people want to have a book published and dont right now. Maybe 2500 of them will get a traditional publishing contract. Half of those will never have more than one book published, and it wont make much money. Most of the remaining ones will never be able to quit their day jobs. Thats what you sign up for when you decide to make a run at professional fiction writing. If that knowledge stops you from writing, then stop and be happy you did. Youve done yourself a favor because the road is hard and full of crushing disappointments. Its no different than acting or music or any other creative endeavor, Many are called, but few achieve anything of note. While you cant be one of the success stories if you quit...you also are statistically unlikely to be one.
Blunt, upsetting, maybe. But like a doctor who tells the cancer patient the bad news so he can prepare himself for what is to come, I honestly think you do need to know that.
Dont stop writing your novel because you are worried that it wont be a bestseller (or be published). Complete it. Edit it. Polish it. If it does not sell now, it might later in your life. At least you will have a book in the drawer for your kids and grandkids to read, and you will have the experience of writing it.
When next in your public library, ask the reference librarian if the library subscribes to magazines about writing and publishing. Some libraries have subscriptions to online versions that you can read at home or on your smart tablet. Some might have the paper copies -- probably in the main branch or, if you live in a large city, the downtown branches where the writers and university students read. Quill & Quire. Editor & Publisher Writers Digest or magazines like them. Read them each month, and search the recent back issues for articles on the business of publishing fantasy novels.
Write your novel first.
What youre currently doing is just fantasizing about being a successful writer. Youre pretending that actually writing the novel is less important than luck or some minor marketing decisions.
Here is something to make you feel like crap. In the same time period where youve been explaining why your novel wont sell and pecking out a few pages, Michael Moorcock would have done some crank and written a novella.
I dont think readers care much about an authors age or experience. Whats important is whether or not the blurb and the cover catches their interest - and if they like the writing when they look inside the book.
Like you say, sometimes theres just no telling what will become bestsellers, and sometimes there doesnt seem to be much logic behind it, but what you can do is to stack the odds in your favour: Write a good book and a fantastic blurb, start building up an author presence online already now and study how to best market your book.
If youre aiming to get traditionally published then you dont have to worry about a cover, but if you arent, then realize you need a good cover. Many dont understand just how important a cover is when it is in fact one of the first selling points of your book. Also, they dont want to pay for one and think the cover is a good place to save money, but imagine a five star quality restaurant that no-one has ever heard of - how many are going to visit if it has a dingy and unkempt facade? Not many. But if it looks good and inviting, then some will give it a try and if the food is good, theyll maybe give positive reviews and recommend the restaurant to others.
No one knows. There isnt a formula for this. I dont think readers care much abt good writing as they do having an entertaining read. Most authors arent great at every single writing element. Your chances depends on how well you market and promote your book. It depends on the market for the book at the time. It depends on if readers will be interested in your story. It doesnt have to be well written to be successful, and Im not even going to address the notion that a book has to be poorly written in order to be successful; its more complex than that, and how good the writing is might not play a role in whether readers like it or not.
No one can give you the complete rundown on how publishing works. Theres a lot of information online from credible sources like agents, editors, authors signed w big publishers, and self-published authors.
Put all thoughts of publication or wealth or fame out of your head until you have a complete, edited, polished novel. That will take you at least a year - probably nearer to several years if youve never done it before.
Broadly speaking, publishers and readers dont care how old you are. They care whether youve written something thats worth their time and money. (Though if youre still young when your first book is published, its possible that the marketing campaign will make something of the fact that youre not yet eligible to serve as President of the USA. But dont worry about that until you have an offer of publication.)