In Action Comics his dropping powers were cited as "cellular decay" that was progressing. Cellular decay is also what makes people look older, and what grays one's hair. So Clark and Lois could be just about the same age, but Clark looks well into his 50s specifically because of the decay.
This then begs the question of how he has black hair again during the War World Saga and Batman/Superman & The Authority special. A friend had the theory that in order to hide his decreasing powers from Mongul he simply dyes his hair black and puts on a show of power when he gets there (and then eventually gets beaten and enslaved). Maybe he gets a temporary power boost back to normal that only holds him for a bit?