While such a move might embarrass some parliaments into lowering their voting age there would certainly be no compulsion. And if it happened this would not necessarily a bad thing. If national parliaments feel embarrassed by the illogic of having differing voting age then it will be up to them to change it. In practice parliaments are unlikely to change their traditions simply because their peers have done so; they will look at all the evidence (which this change would provide more of) and then decide the best way forward for their democracy.