There is simply no denying that wide age disparity has a considerable impact on the older player, esp when the older man is in his mid-30s, as in the case of 34-year-old Lin Dan and 35-year-old LCW against Viktor Axelsen, 11 and 12 years younger respectively. However, if Lin Dan is at 30 years old and LCW, 31, that is four years ago when VA is only 19, then I'd venture to say both the old maestros would still be able to handle the relatively inexperienced VA. I mean, at the age of around 30, Lin Dan and LCW have proven to be more than a handful for all the younger ones, and they have the results to show. The clearest example is the then 36-year-old Lee Hyun Il who beat VA twice and JOJ once last year but failed to clinch any title as he was beaten in one at the semifinals to Son Wan Ho and the other to Shi Yuqi in the final. And this year at the KOR Open just a month ago, the now remarkable 37-year-old LHI managed to beat the young hard-hitting 22-year-old Wang Tzu Wei in R1 in a grueling close-fought three-setter but was stopped the next round by his compatriot SWH again. In fact, exceptionally few players can be at the top past 30, in recent decades we have only Lin Dan, LCW and Zhang Ning. Lee Hyun Il while very much still on top of his game in his early 30s is admittedly not at the highest level on par with the two aforementioned male counterparts. More or less the same goes for Peter Gade who retired in 2012 after LOG'12 at age 36. So if I may used the three examples I've given, I'd say age 33 +/- one is about the age when a top athlete is considered too old to maintain dominance. That is not to say as medical science advances, the age limit may not be pushed higher in future. What about late bloomers, like Sho Sasaki (retired in 2017 at age 35), the now 36-year-old Hu Yun, and the 31-year-old Brice Leverdez ? At most I'd group them with Lee Hyun Il though he's not considered a late bloomer (more like evergreen), in which case I think 33/34 is the maximum age at which they could be on top of their game and then decline sets in fairly quickly. All in all, I think 33 may be the cut-off date for any athlete, evergreens and late bloomers, but, of course, I cannot be too definite about it and qualify my statement accordingly (and, I add, doing a scientific study is beyond me).
As for determining whether a particular player is on the up or down or at peak, it's at best a judgement call based on results, but it'd be better to wait years later and look back with the benefit of hindsight than to use arbitrary assumptions as different player rise and fall differently and also some players' form tend to fluctuate more than others. Moreover, we may also have to take into account child prodigies and late bloomers who don't fit the conventional career path of the majority of players.
