U/s measurements are notoriously inaccurate. And no, his size is not a determinate of his age. My little sister is bigger than me, but not older. Each baby grows at different rates.
My experience was that my 36 week growth u/s said DS was already 7lbs. Even the tech said that there was no way he was that big already. He was born at 40 week weighing 7.5lbs, Don't let the numbers worry you at all.