Inter's squad is not as old as people make it out to be.
Their main players are: Lautaro (27), Thuram (27), Calhanoglu (30), Barella (27), Dimarco (27), Dumfries (28), Bastoni (25).
Pavard is also in his best years (28).
Mkhitaryan, the keeper and Acerbi/De Vrij are older indeed, but that's 2-3 players that need replacing.
Even on their bench they have young players: Frattesi (25), Carlos Augusto (25), Bisseck (24), Zalewski (22), or some who are still in their best years, like Zielinski (30). They have some older ones too, like Darmian, Arnautovic, and Taremi.
This is fixable in a single mercato.