“The hard part is getting to the top of page one.”—Tom Stoppard.
That's it. Start now. There is a lot to be said about technique and process and the art of what's interesting, but you'll never learn any of these things if you don't actually just start writing.
One of the best favors I ever did myself was decide to start writing at sixteen. I liked sitting down with something to drink and some music, and making something that wasn't there before. I even managed to bang together a few coherent sentences, I think. Moving on from writing isolated scenes that didn't go anywhere, I started a bigger project that didn't go anywhere either, but I was writing, and writing was fun enough. A year and a half later I pieced together a short story that wasn't very good, but it was the sort of thing I wanted to read but couldn't find, and it had potential. That story was the beginning of my first novel.
Now, at twenty-six, I've re-written that novel three times, revised it even more than that, and worked on a bunch of other projects along the way. After everything, and with much help, the novel is fit to read, and I'm even happy with how it turned out.
If that time frame is enough to scare you off, then you want to write for the wrong reasons. But that might be unfair of me. It may not take you ten years to write something good. Those weren't ten years of nose-to-the-grindstone writing, and starting at sixteen is its own disadvantage in terms of life experience. Your trajectory will undoubtedly be different than mine.
Age is not the issue either way. You can't be too early, or too late. My friend, who is sixty-five now and a retired teacher, has been walking this writing path alongside me the past eight years. The fear of wasted time weighs heavier on him than me, but he's getting there all the same, in spite of the doubts.
Whatever it is that you want to do, if it's write a novel, or a podcast, or make a comic, or open a bakery, you're never going to know everything you need to know. You're never going to get any closer by daydreaming about it. The fastest, most effective way to learn is to do. This doesn't mean you have to sit down at chapter 1, page 1, “It was a dark and stormy night...”, and start sprinting through the pages. But decide what you need to do to get moving, and then do that. If you want to sketch out a few plans, then sketch them out, but start.
The hours add up. You learn. You get better.
I'm not as good of a writer as I want to be, but when I look back at work I wrote two years ago, five years ago, I see how far I've come. I don't know if I can say I understand this whole writing thing, but I understand it better than I did.
When you sit down to write, the tendency is to do it to find out if you're any good—if you have some hidden writing aptitude. But the reality is if you like writing at all, if you're interested, if you want to make things, that's all the aptitude you need. It's always hard at first. The words are slow and strange. Sit down knowing you're not going to get it right the first time through. Sit down, instead, with a mind to learn. Take your small victories to heart. Be proud of your time spent working, but know you still have more to do and farther to go. Know that tomorrow you'll always be glad you started now.