Can someone give some perspective to the non-football follower here?
At what age do players of this caliber and these positions begin to slow down? How many more elite seasons can be expected? Are both likely to still be around and be high end players at the next World Cup? I recall hearing Ronaldo tied a record for number of different World Cups scored in, so I'd guess he might be running out of track pretty soon.
Although I agree with cgf's answer, I'd like to expand upon it.
Historically, once a player reaches 29 or 30 the expectation is their careers will trend downwards. So we've reached the stage where we should treasure every last moment of Messi and Ronaldo, because either or both of them could go off like a stale cheese at any moment. But the sport has changed so much uncertainties reign. On the one hand, sports science has never been more sophisticated. On the other, top players have never before been asked to play so many games- and there's a mental effort as well as a physical one required to retain the motivation to stay on the treadmill.
And as cgf has hinted at, however highly we rate Messi and Ronaldo, even they are still mere individuals in a team game. Ironically, while we might expect a player to be at their peak between the ages of roughly 24 to 29, Messi and Ronaldo are united in having won more team honours either earlier or later than that. Having been magnificent in 2010-11, Barcelona failed successfully to evolve their team from 2011 to 2014, so Messi found more and more was being asked of him to the point that by the 2014 World Cup, he appeared jaded.
For Ronaldo, meanwhile, 2009-2013 was close to a lost era in terms of big team prizes. Again, the team around him wasn't right. So even if we rank him as the Greatest Great of All the Great Greats, that's an awfully long chunk of time where we have to say, 'Well, you know, one chap can't carry ten people on his own'. The last five years may have been the most successful of his career, but I defy anyone to persuade me that's because he's played the best football of his life. It's been because at club level, football has chosen to ape France circa 1675, with the few having a jolly old time strangling the life out of the many. In this environment Real Madrid, as the footballing PR-wing of Spanish centralism, enjoy a practically consequence-free bottomless pit of money that allows them to build a squad of remarkable strength. (Barcelona, being the footballing PR-wing of Catalonia, also enjoy economic support that goes beyond what normal football clubs expect. The point is that for clubs from everywhere else in Europe- Real Madrid is pretty much beyond being a football club anymore).
I referred to Madrid's
squad, mark you. Successive European Cup finals in which Gareth Bale starts on the bench says it all. Ronaldo is a footballer who belongs in the all-time pantheon who has reached an age where he benefits from playing football less. Note that in these last five years, he has played 50 club games in a season once - ironically that was a season in which Madrid failed to win La Liga, the Copa del Rey, or the European Cup despite Ronaldo scoring more goals than before or since.
And even then, such are the demands on players that that magnificent physique of Ronaldo's gave up on him on one of the most important occasions of his life - the 2016 Euro final. (Admittedly, if we're to believe some of the posters around here he defeated the French by assaulting them with his All-Time Great Brain Waves to the extent that they turned to jelly when a bloke who used to play for Swansea bore down on their goal. Who knows, maybe in time we'll discover that North Korea's new-found engagement was borne out of Ronaldo willing world peace).
In those same five years, Messi has surpassed 50 games three times. Barcelona has dominated La Liga in that stretch, while Madrid- asking Ronaldo to do less in the league, has made the European Cup nigh-on its own.
Given those stats, I think its worth asking whether Barcelona- who during those five years have lacked a top-class coach- haven't handled Messi as smartly as they might. Nor has playing the World Cup in 2014, then Copa America in 2015 and 2016 left him with a lot of breathing space. (It bears consideration that once upon a time the Copa America was biennial, only to become a four-yearly event with the specific aim of relieving star players overburdened by a crowded calendar).
In short, I'd be amazed if either player played in 2022, let alone dazzled. But depending on events over the next four years one or both might usefully be accommodated in a team that employed them thoughtfully. Whatever reservations I might have about Zidane as a coach, we're indebted to him for his smart deployment of Ronaldo. Whether Messi will be as sensitively used in the future is a moot point. But so far, the portents aren't good.