these are very general questions which are difficult to answer.
The one regarding the age of the ships cannot be answered at all, as Carnival has a very diverse fleet. Their oldest ship is 20 years old, their latest only a couple of months - plus they have more new ships on order. So it's really impossible to answer that question.
To get a general overview of Carnival, why not read the reviews on cruisereviews.com? As Raoul said, some of their ships are a bit long in the tooth, but many are quite new. You may find one that fits your style among them.
As it was once described to me by a travel agent, Carnival is more "las vegas" and rccl is more "brass and class".
I just got back from Carnival after being on rccl a couple of times and ncl and celebrity. My personal feedback is that I was not impressed with the decor on Carnival (way too loud, vibrant, glitzy), but also the food and service did not hold a candle to that of rccl.
Again, that is my one experience on Carnival, but from now on I'll continue my cruises with rccl.
I agree about the Las Vegas look on Carnival. Although I like RCCL's decor better, I think it's still more glitzy than, say, Princess. As far as other aspects of Carnival, I have had food ranging from pretty good to excellent on their cruises. Food, I find, is a highly subjective issue among cruisers!