Do you seriously think an ATG 5-6 years older could have inflicted any real dent on Federer's slam count ? Remember this is not A Federer vs Djokovic scenario, both Fed and Novak are a bit from the modern baseline era, the string tech they played with, their decline in 30s being slow, lot of things do work in sync... but the guy who is 5-6 years older to Federer would have been 1975-76 born like Tim Henman/Philipoussis/Kuerten, these guys were less fitter than Fed's age group and Fed's age group was less fit than Nole's age group for a reason, the evolution of tennis was such that Fed had an advantage over those 5-6 years older to him in a way that Nole had over Fed, maybe even to a greater degree I must say. Thats why I don't think someone 5-6 years older at 28 years old was ever gonna do any dent to Fed at 22-23 who was on an uptrend while the older guy would be on a downtrend and having to change racuqets to adapt, just not happening.
'
Same aged ATG is a legit argument, someone like that could have taken lot of slams from Federer but someone like that would have made life for Djoko-dal harder too, at least until the early 2010s.
Had Sampras-Agassi been 5-6 years older only, even then Fed would have wo bulk of the slams from 2004, see Pete declined a lot after 1998, isn't it ? Do the same math for 5-6 years after that, now 1998 for Pete would be his 2003, he would have to adapt to new strings, something he did not want to do. I dont think this benefits Pete at all, Fed could lose a slam or 2 but Pete loses more.
Djokovic had it super hard in his 20s, agreed, but some of his problems was also because he was not good enough. Look at HCs, Novak started blasting through everyone at the AO and his dominance was never in doubt. Why is that? Because HCs are his surface and he really is great on that. However if you look at Grass, this is a surface where Nole is struggling in his 20s because he was actually bad (ATG standards) on this surface until Boris Becker became his mentor. You cannot blame competition for this, that alone was not the problem on Grass. On Clay, Nadal was there, we can agree that was an uphill battle always but not on Grass, that surface Nole's problems were because of his own shortcomings. At the US open too he lost plenty in that 12-14 period, can that be blamed on hard competition alone? Would a 10 year gap between him and Fed have helped Nole vs Murray/Nishikori? Surely not, and these are not great ATG players, these are players like Roddick/Hewitt types.