still persisting with this magical 30 when players suddenly turn trash? Players nowadays perform well into their 30s. Could easily see another 4 prime seasons from Bobby Firmino, Salah and Mane.
Yes, that is the magical number when players start declining. In fact, most elite footballers start declining even earlier.
You talk as if players like Bale, and Rooney at United, and Kante (29), and Countinho (28), and Aguero for a few years already (32), Thiago (29), Kroos (30), David de Gea (29), James Rodriguez (29), Cesar Azpilicueta since last year (31) and tons of others are from a different generation.
Some players age earlier than others. Some players like Ronaldo and Messi are still world-class in their mid-30s, but that is because their peak (ceiling) was infinitesimally higher than that of an average footballer. And by the way, both declined after they passed their 30s.
Salah and Mane may still have 4 years of top football, but that doesn't mean they won't have declined in 4 years from their current peaks (or of last few years).