Even though he was never slain by the Blades, he shouldn't look so old compared to other dragons, especially considering he retired to the Throat of the World for most of the time between Alduin's banishment and the 4th Era. Other dragons also survived for this time, including Mirmulnir (first dragon the Dovahkiin fights), and Vulthuryol (who was hiding in Blackreach the entire time). It's possible he received his aged appearance from fights with other dragons when he defected and aided mortals, but I believe that immortal creatures such as dragons would heal over time, especially over thousands of years between the Dragon Wars and Skyrim's Civil War.

Also, I acknowledge that his appearance helps players identify him, like Alduin's unique look, and Odahviing's red coloration. However, I'm looking more for an in-universe possibility.

Because he's ancient. Dragons are immortal. He's been there since the beginning. Not to mention that since he he opposed Alduin, he likely fought other Dragons.

He is immortal, but I've never seen anything that says he is unaging. He was with Alduin; his brother, iirc. He is ancient. Dovah are ambitious and driven to dominate; Paarthuranax means "Ambition Overlord Cruelty" even before he rebelled against Alduin; he's gonna collect scars fighting humans and other dragons.