If natural fitness is worded how you mentioned for researchers then it needs some rewording, as in-game the attribute lengthens how long a player maintains their top CA ‘past their prime’.
In this context, Wayne Rooney's came in to the prem at around 16 years old? and left around 32 or 33? What an exceptional player he was by the way. However, it's plausible to debate whether he reached his peak by 28 or 29, given his extended tenure in the league from a very young age—around 10 or so years playing at his peak lets say.
Then to still consistently perform at the highest level week after week, month after month, and season after season—is a remarkable feat. Despite potentially passing his peak, the fact that he sustained such a high level of play for an additional 4, 5, or even 6 years beyond that phase speaks volumes about his exceptional natural fitness, wouldn't you agree?