I guess the only way Djokovic fans will ever understand the age differential is when it happens to their guy.
In the past four years Roger has been 35-39 years old, playing a dominant #1 player six years younger. If Thiem suddenly grows a pair and becomes a truly great big match player (doubtful), let's watch the H2H between Djokovic when he's 35+ against a player in his late 20's. If Thiem and Djokovic play 20 times after Djokovic turns 35, the H2H would likely be 18-2 for Thiem. It's utterly immaterial since they're not generational rivals and few care about H2H outside TTW. But when Novak fans endlessly play the H2H card it's ridiculous, since both of his H2H's against Fed and Rafa are a razor-thin margin.
Just a little study of history would instantly show that for all ATGs who continue to play for a long time H2Hs always are bad looking when compared with other ATGs. The reason is obvious. When you have a young player who is all of a sudden on fire that happens rather quickly. Think of Connors, Borg, Mac, Becker, Edberg, Sampras, Fed, Nadal, Djokovic.
It did not take these guys years to hit a very high level, so there were only a few early years between bursting on the scene and totally dominating. And usually young players don't even play peaking ATGs until they establish themselves. You will hear again and again about how Connors crushed Rosewall, without even considering that Jimmy was born in 52 but Rosewall in 34. Borg against Laver was 56 against 38. Connors and Mac, 52 and 59. It's pretty obvious.
So you have these young lions and in just a few years they are peaking, while declines happens slowly and can go on for many years.
You just can't explain to fans what happens when players age until it happens to their faves and they experience it. I'll never get of the shock I felt when watching what happened to Borg between 80 and 81.
The ATGs who go on playing well after age 30 often seem to be almost ageless, and then the decline happens so fast it blows your mind.
With guys who have unbelievable defensive skills return can never go anywhere but down with age, so the only compensation is to win more on serve. Both Nadal and Djokovic have pretty much maximized their service games, so the moment return stats fall they will be done. It's just a matter of when it happens. Fed had nowhere to go, and people do not understand that. His serve was so good early on that the only way he could maintain that same high level was to keep his return skills equally high but with a racket that was unfriendly to anyone much over age 25.