If we assume that most of them die off at 35-45, then 40 isn't the equivalent of a human at 60-65. It's equivalent to 70-75. While it's true that Christopher Lee is pretty sprightly for a guy in his 80's, let's not pretend that he's the average.
Runner-class Orks are also far from average. Even one stat at 5 or higher puts a person in the "Very exceptional" category in terms of overall humanity. If body approximates constitution, Orks probably never get "decrepit". These guys are usually martial artists of some sort, and have all types of genetic & lifestyle advantages over the "average ork". Sure their mom & dad could be long gone by the time they're 45, but that doesn't mean that they have one foot in the grave and need bifocals. In many ways, a human life is full of examples of "use it or lose it" and that includes your life overall. I'd go into gruesome statistical detail but that's not necessary. Point is, overall, the more of their brain, body, instincts, vision, strength, etc. that the Ork uses over their lifespan without "retiring" the more likely they are to go kicking & screaming to their death.
If Average Lifespan of a human is 75, and humans live to 120+ on occasion (that's an additional adult lifetime!), a PC-caliber ork being an exception to every rule should be able to make it to 75 (perhaps even more) and still be an active member of the community.