Why? Redemption required blood sacrifice. Because Jesus was the perfect sacrifice (Lamb of God), there is no more need for animal sacrifice. When that which is perfect comes, that which is imperfect will be done away. Anyway, since He is called the Lamb of God slain before the foundation of the world, the point is moot anyway. The only way He would have died of old age would have been to defy the Father, which He wouldn't have done, because He was one with the Father. He knew the purpose for which He had come. Everything in His life pointed to His death on the cross. He would rather there had been another way, but I don't think it ever occurred to Him to bail out.