I'm one of those writers who figured out what I wanted to do very early. Age 9, to be exact. I didn't start doing my own writing right away, however. I read a lot first, and penned (literally) my first short story at 11. What can I say? I'm the type who learns best from hands-on experience. Back when I was 11, I set a goal for myself that seemed incredibly reasonable at the time. I told myself I would be a published author by the age of 18.
It was a perfectly logical plan...for an 11-year-old. I'm past the age of 18 now, good and past it really, and sometimes I still find myself setting ridiculous goals for no reason other than to make myself feel like a failure.
I mean, that's probably not why I do it. I don't think I actually set myself up to fail, but this is almost always the case. I didn't get published before 18. I didn't get published before 20. It took me years to get published, and even longer to learn that getting published isn't nearly enough. You have to keep getting published, and selling books, and you have to do it a lot.
And I've learned that trying to set these weird limits for myself (by this age I'm going to... Before I turn this age, I will have done...) just doesn't work. Making unreasonable demands on yourself will only make you feel as though you've failed if you don't meet that goal. If you publish a book, if you sell a single copy, if you have just one fan you haven't failed. It's hard to remember that if you're not hitting a mark on a calendar that you set for yourself out of some wellspring of optimism.
Be optimistic. Try to reach goals. Aim high and keep trying. But don't give yourself a time limit. Don't put a date on your success. When you do, it only diminishes every small accomplishment you do achieve. You don't have to beat the clock or predict the future. Just write, and that will be enough.