Re: Where are teenagers ?
Perhaps it has to with the fact that the average male fully develops at 23 and once they are fully developed and can hone their game it takes on average about 4 years to establish yourself inside the top 100. Not every top 100 player was a junior star, not every top player had an ATP ranking before they were 18 etc.
Another reason could be the fact that Tennis players now realise they have to do something with their life (ie. go to College ala Isner) before they go pro. Perhaps it is the fact that 27/28 is the age where players hit their physical peak and therefore hit their ranking peak and then start to decline physically from then onwards.
That may explain it, but it is a very strange statistic, Maybe the stress on young body from the more travel professional players do these days has taken them a longer time to get up to the professional standards?