It lists 35-55 as the average lifespan; it also lists humans' average lifespan as 65. There's nothing in SR4A about rapid-aging, or reaching puberty at a different age from humans. The implication, IMO, is that Orks simply die younger than humans do, due to poor living conditions/violent lifestyle.
All of Shadowrun 4 is compatible with former releases of Shadowrun (c.f. the online FAQ). The early puberty/physical maturity of Orks has been mentioned as being one of the big factors in Ork---uh---self-control issues. It may be in DNA/DOA -- yep:
p. 32 (Welcome to Wilhem Park)
Orks receive much of their bad press from the fact that their adolescent males tend to be more "active." Physically, Orks reach maturity at about age 12, while a human does so at 18 or so. Though an Ork reaches an advanced state of physical development earlier, he still has the emotional maturity of a 12-year-old. (Put a 12-year-old in the body of a body-builder to imagine what happens!) A vernerable, old Ork is one who has reached the ripe age of 35, and he will begin to show signs of aging at about 20 years."
So that's a hint to where some of us may be getting our information -- and it has yet to be refuted by either RAW or canon, to my knowledge.
I'm not saying that it is - what I said is that the books currently in print do not mention early puberty for orks,
so I feel more comfortable ignoring the concept. I'm free to do so at my own table anyway, but as someone who generally prefers to follow canon, the fact that this concept has remained largely untouched and unmentioned for at least an edition or two gives me a bit of extra justification, at least by my own standards.
As to why I would want to ignore it...well, it makes the whole game feel more ridiculous to me. It also makes any human/dwarf/elf who is dating an ork a likely pedophile/pederast, and calls into question how the education system would work, and why Orks have an intelligence penalty when in fact they would be at the prime of their learning abilities at a (relatively) younger age - all Ork children would be geniuses (compared to their human/elven classmates) who gradually flatten out into morons by adulthood, which is kind of silly when you think about it.
Socio-economic explanations for their shorter average lifespan seem much more logical than a physiological explanation.