Talk about his stats sliding, sure, but the armchair analysis of his age based on his looks or voice (!) is dopey. People are different.
Here are pics of Serge, Greg Oden, and Lebron all in their 26/27 y.o. seasons. Are we sure Serge is a few years older than he thinks/claims he is b/c he "looks/sounds old":
He looks younger than both those guys did.