I believe median time to unicorn status is now about 7 years, so this is not as uncertain as you think. In other words, 50% of YC's 7-year old unicorns have already showed up. The number is higher for older companies.
Yes, but average time to unicorn status has been increasing since the time of Dropbox and Airbnb. I assume you can draw more inferences from companies that are earlier-stage, too.