It is not like that.
I always say a generation is 10+ years and not 5 years, there is a reason for this. Roddick is Federer's peer in his generation but Djoker & Nadal are also in Federer's generation but they are the younger lot of players in the generation. So someone born in the early 1990s is a flat 1 generation after Federer.
Now notice how Medvedev/Zverev/Tsitsipas are better as a trio than Thiem/Dimitrov/Raonic, the younger lot of guys born in 1990s gen are better than the older lot born in 1990s gen ..... this is because the older lot are closer in age to the Big 3, the closer you are the bigger loser you become.....
So unless you have a full 10 years apart from a player, you will suffer ..... Thiem did not suffer at the hands of Federer because his age gap was 11-12 to him but he suffered to Nadal because age gap is only 6-7 years which is nothing.....
This whole myth of decline, old age, generation gap being 5 years is a concept restricted to 1990s or before that, not for current athletes..... So 1990s generation being bad is a side effect of 1980s generation being so good..... It had to happen...... Djokovic suffered in his 20s because he was the youngest of the 1980s generation and hence the biggest rookie, but in his 30s he has compensated for that by being the slowest to decline, not just because he is youngest of his 80s gen, but because he maintains his body the best and he is the best of the 80s physically,........ so
his physicality has made him the best
@DSH ... you cannot blame 1990s gen being bad.