Depends on motivation. When Roger Milla participated in the 1994 FIFA World Cup, he was 42 years old (and he scored goals in that tournament as well). Ronaldo may still want one good performance before he hangs up his boots. But then, what about Messi? He won FIFA World Cup as well as Copa America for Argentina. So what's left to achieve? I would suggest that he would be fine if he focus on Major League Soccer and earn himself a few hundred million USD as the retirement package.
Messi has actually achieved everything in his career, including being able to win the World Cup, in terms of ambition he may not have thought about it anymore because he has felt everything. And it is also possible that he has started to consider retiring in the near future, yes that might happen. We are just speculating here, because previously we had discussed the possibility of 2022 being the last World Cup for these two players.