No not at all, was Rooney not world class when we signed him? If anything in Rooney's case the potential he showed during his early days at Everton and at Euro 2004 was greater than what he's produced over the past two seasons
Aguero clearly has world class ability thus he's a world class player, just because he hasn't had long enough to display that talent doesn't negate that fact.
Look, I'll admit that assigning someone with the label, "world class", is not exactly an empirical science. It's largely subjective, and it's almost always arguable, but there are certain indicators that most people look for.
Having great talent is not usually enough to warrant the label of world class, even if you are considered as one of the most talented players in the world. Plenty of players have been extremely talented throughout football history, but have been unable to express that talent when it really mattered. Ricardo Quaresma is possibly more "talented" than Ronaldo, but only one of them can be considered as anywhere near world class. There has to be certain standards that a player reaches before most people are prepared to consider them as being one of the top 5 or 10 players in the world — or one of the top 2 or 3 players in their position — which is what world class essentially means.
Lots of players have looked potentially world class at clubs that never compete within the top echelons of world football, so the real deciding factor has to be whether they can replicate their form, and express their undoubted ability, in the pressure moments of the biggest games — usually at one one of the biggest clubs in the world, and in the later stages of the Champions League (and often for more than one season, as well), as well as/or in the later stages of one of the major international tournaments.
Whether that is always fair or not, is arguable, but you'll find that it is usually only those players that are, not simply talked about as being one of the best, but are thought to have confirmed it.
So, I suppose that it's fine to talk about a player having world class talent — i.e. being one of the most talented strikers/midfielders/etc in the world — although I'd still argue that point where Agüero is concerned, as he is still only 21 years old, and has had 2 years at Atlético where he has a goal ratio of 1-in-2, which is undoubtedly impressive. But to consider him as a world class player is going to require him to move to a bigger club, and to compete at the very highest level, either with that new club, or with the Argentinian national team. Otherwise, there will always be that nagging doubt in peoples minds about whether he can really reproduce the Atlético form, and affect the biggest games of players career. We'll see.