Hm, not counting immortals...? That makes it hard. Namor's not immortal but he's pretty old. May be him. Probably him, really, unless you count that as 'extended life span.'
Due to the reboot, most of DC's are out of the running, everyone's super-young nowadays. Pre-reboot, Jay Garrick and Alan Scott were the shoe-ins.
Judge Dredd is interesting in that it's run real-time since the late 70s, and the main character didn't start young.
Sentry has been around since the late 50s and retroactively taught like every super hero everything.
His immortality hasn't really factored in to that though so I wouldn't consider it. Same as Namor. However unlike Namor and Wolverine he was a hero the entire time until Seige really, well not evil anyway, just an old man
Lesse, other books... Fred Perry's Gold Digger has Theo Diggers, who's I think in his 70s nowadays. He's not super-active, but he does return to the field occasionally in big throw-downs, and as a mage that's fine. A lot of the old-gen heroes in GD are fairly old, often originally active in the 50s/60s, before a huge incident convinced a lot of them to 'retire' (by which I mean, many left the public eye but still hero). Debra and Miracle and Xane are likely disqualified for either not aging or aging slowly, but Justice Beetle and others are in.
2000ad has steadily aged him. He's had treatments to keep him in fighting shape, but time has passed year-for-year from the beginning.