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Don't these two statements contradict each other?

Good catch, and the first of those two statements may be a bit misleading. Though in this case, "young" is a relative term.

The book actually suggests that all (non-half) races of Eberron were created in the immeasurably distant past, during the Age of Dragons. But it was a long time before the common races became civilized. It didn't happen during the age of Demons, which lasted from over 10,000,000 years ago to about 100,000 years ago (when the couatls sacrificed themselves to bind the rakshasas and other fiends in Khyber). The first steps away from primitive existence happened during the Age of Giants, when elves started to become a civilized race (as the slaves of giants, about 80,000 years ago). During the Age of Monsters, the civilizations of the other common races started to rise.

It's not stated when human civilization started on Sarlona (the continent now ruled by the Inspired), but humans didn't actually come to Khorvaire until Sarlona until 3,000 years before the campaign setting's start year (and don't seem to have a significant amount of recorded pre-Khorvaire history). Compare that to the other races:

-80,000: Elven civilization begins in Xen'drik
-38,000: Goblinoid kingdoms rise in Khorvaire
-30,000: Orc nations rise in Khorviare
-12,000: Dwarves migrate from Frostfell to establish a civilization in Khorvaire

No dates are given for the rise of halfling and gnome civilization, but gnomes have been in Zilargo for "thousands of years" and apparently clashed with the goblins back when they were an empire. Halflings have been around for a long time, though they've been in a more "primitive" state for most of that time. Still, the Mark of Hospitality appeared among the Talenta halflings 200 years before humans even arrived, indicating they were sentient and establishing a history for a reasonable amount of time beforehand.

So human civilization as people in Eberron know it is pretty young; the humans are definitely the most recent (non-halfbreed) race to arrive on Khorvaire. Half-orcs, half-elves, and (I assume) shifters have been around for the past 2,000 or so years. Since about 1,500 years ago, they've had communities of their own.

The distinction I was trying to make was that in many campaign settings, half-races are very rare and usually a concept that has emerged quite recently. In Eberron, they're true-breeding, and they've been on Khorvaire almost as long as humans.

Hope that clears up any confusion!