I think they're the oldest average age in the Premier League. But it's not just that, it's that most of their key players are at their peak now, so while they've probably still got a few years left with most of the squad, basically they're going to all need replacing at the same time.
Milner, Adrian 34
Henderson and Wijnaldum 30
Van Dijk, Matip, Firmino, Thiago, Shaqiri 29
Allison, Mane, Salah 28
Fabinho, Oxlade-Chamberlain 27
Robertson, Minamino 26
Keita, Origi 25
Jota, Tsimikas 24
Gomez, Phillips 23
Alexander Arnold 22
Jones, Williams 20
Their younger players aren't really regularly getting game time, and they're not actually that young anyway. So it depends what sort of appetite the Americans have for another massive spending spree when they're all in their early 30s.
Compare that to us:
Fernandinho 35
Aguero 32
Walker, Gundogan 30
De Bruyne, Mahrez 29
Ederson 27
Stones, Sterling, Bernardo, Laporte, Cancelo, Mendy 26
Ake, Steffan 25
Zinchenko, Rodri 24
Jesus, Dias 23
Torres, Foden, Garcia 20
A far more even spread meaning that yes, we need to replace Aguero, but for most of the other players on their way out, there's already a younger alternative that's actually getting regular game time. 8 of our starting eleven that beat them yesterday was 26 or under, and we brought on a 23 year old.