I don't agree that stars in the Belgium team are too old. De Bruyne is 31, Curtois is 30, Lukaku 29... All of them are around 30 years of age and that is the time when a football player should still play at their best. Especially for the star players like the ones I mentioned. OK, Mertens is a bit old with 35 but he is one of rare players that old in the team.
In terms of age indeed De Bruyne, Curtois and Romelu Lukaku are getting older, but these three players are key players for the current Belgian team, the experience of these three players can make a difference for the Belgian squad. The biggest problem for Belgium is whether Romelu Lukaku will recover before the World Cup takes place, because there are still many who doubt his recovery ahead of the World Cup.