Technically, a professional athlete is anyone who gets paid for their sporting performance. There is a difference between certain professionals, primarily the semi-professional and professional sportspeople.
The Semi-Pro Athlete
Usually does their sport part time whilst working another job, studying, parenting etc… Semi-pro athletes are paid considerably less than their professional counterparts, not enough to live off solely, forcing the need for other income sources.
Usually work from limited term contractual agreements with sports clubs and associations. Their financial remuneration from their sporting performance is enough for them to live off.
In many occasions the big names in sports are observed with an innate sporting ability early. Anyone that has viewed a few clips of athletes on YouTube have probably seen footage similar to some of these examples.
Lionel Messi as a kid.
Here’s an example of a present day talented kid, Jordan McCabe.
From these videos it’s quite obvious to tell that these young kids are special talents and with the right nurturing could turn into highly successful professional athletes. Video footage is a modern day supplementary to the physical scouting process. It doesn’t remove that function altogether but it’s definately an highly effective way to get noticed if exposed to the right people.
Most athletes start at a very young age, not necessarily in the sport they are to compete in later, but they are active and engaged in the development of their physicality and technical motor skills. Many athletes go through youth development programs in their particular sports, these programs attempt to identify talented youngsters as early as possible and get them into a dedicated athlete development structure. Here’s a good article on some top tier Football academies around the world (source: http://bleacherreport.com).
So the earlier the better essentially, that being said there are plenty of examples of late bloomers who make their way to the professional level without going through formal academies and representative juniors. The semi-pro level is a fairly standard stepping stone for a lot of athletes that don’t necessarily take the academy or college matriculation path.