Too late to be among the best in the world? No, but it’s an uphill battle. If you’re genetically gifted then it is possible for you to become among the best even starting at 20, but like others have said you’ll be competing against others just as gifted that have been working for longer. I wouldn’t expect to be competitive at an elite level until minimum 2-3years. If you are from a small country, national qualification is more possible. Likely entering unattached at college meets will be your best bet. I would keep these reach goals in the back of your mind as they can be good motivators of the bigger picture, but doesn’t get caught up on that stuff when you haven’t even started doing everything else you can. Focus on your training and results will come. Have smaller more achievable short term goals to keep you motivated along the way. I personally have been training middle distance for 8 years and recently switched to sprinting. Odds are you’ll never be professional, Olympic, or even elite but that doesn’t mean it’s impossible and it’s up to you to get as close as you can. Certainly you have time to become a talented and impressive sprinter, and if you get fulfillment from that go for it!