He was considered the world strongest even with his age because no news of a defeat had been reported. We didn't even saw his most recent fight (before the war) against Shanks.
It's possible that if they fought all out now WB would loose.
To the characters in the OPworld, sure. But to the readers it was made exceedingly clear that Whitebeard being stronger than everyone else, even while ill and old, was a cold, hard fact. Not only is there absolutely nothing that suggests he wasn't the strongest, there is also the fact that his introduction box stated he was the strongest. Those boxes have never once been wrong, and if anything seem like the word of Oda.
So until we get some evidence that may suggest otherwise, he was the strongest. That's unlikely to happen though seeing as how he's dead. We can use Mihawk as an example though, we currently have no reason to assume he isn't the strongest swordsman and we shouldn't question it until the manga starts to suggest that he may not be.
I've always thought of Shanks to be the strongest pirate in the world, second only to WB and now, potentially, Blackbeard. Of course, then comes the two other Yonkou, who should be unbelievably powerful as well, their level of power either equaling or being a slight level below Shanks' own.
That's only what I think, though; we still have yet to see how strong Big Mom and Kaidou are, although, to be honest, Big Mom seems to interest me slightly more..:P
Oda has definitely been hyping Shanks up, I wouldn't be surprised if he is the strongest pirate currently around right now. Although, I do think Mihawk is in all likelihood stronger.