To put those years in context, here's the year-end age for various players:
2013: Federer 32, Nadal 27, Djokovic & Murray 26, Del Potro 25, Raonic & Janowicz 23, Dimitrov 22, Tomic 21, Kyrgios 18, Coric, Quinzi, Garin 17
2014: Federer 33, Nadal 28, Djokovic & Murray 27, Del Potro 26, Raonic & Janowicz 24, Dimitrov 23, Tomic 22, Kyrgios 19, Coric, Quinzi, Garin 18
2015: Federer 34, Nadal 29, Djokovic & Murray 28, Del Potro 27, Raonic & Janowicz 25, Dimitrov 24, Tomic 23, Kyrgios 20, Coric, Quinzi, Garin 19
2016: Federer 35, Nadal 30, Djokovic & Murray 29, Del Potro 28, Raonic & Janowicz 26, Dimitrov 25, Tomic 24, Kyrgios 21, Coric, Quinzi, Garin 20
2017: Federer 36, Nadal 31, Djokovic & Murray 30, Del Potro 29, Raonic & Janowicz 27, Dimitrov 26, Tomic 25, Kyrgios 22, Coric, Quinzi, Garin 21
2018: Federer 37, Nadal 32, Djokovic & Murray 31, Del Potro 30, Raonic & Janowicz 28, Dimitrov 27, Tomic 26, Kyrgios 23, Coric, Quinzi, Garin 22
2017 seems to be the most likely year that the keys to the castle are passed along to the younger generation, with their new reign firmly established by 28. 2016 should be a lot of fun - as I wrote, "anything goes." The current Big Four will likely be in serious decline, the next generation in their primes, and the young pups starting to show up.