Well, if the problem were "not enough resources", then you would expect that calories consumed would positively correlate with lifespan. But of course, calorie

Well, if the problem were "not enough resources", then you would expect that calories consumed would positively correlate with lifespan. But of course, calorie restriction increases lifespan. This theory was rather popular before 1950 and part of why people found calorie restriction's success surprising. A more modern and sophisticated version is called the "disposable soma" theory of aging, which states that reproduction competes with maintenance activities for the body's resources. But it too suffers from the same problem, and the additional problem that some organisms do not appreciably age. Yes, you are right that lifespan correlates with body mass/volume among species quite nicely, with some exceptions. And it is indeed quite suspicious that aging begins just about the same time growth stops. Growth hormone (GH) levels drop with age and GH supplementation can slow some age-related phenotypes. But, on the other hand, GH-deficient mice have increased lifespan, and GH is induced by cancer and its supplementation may increase cancer risk. There is a similar story for other growth hormones like IGF-1. Of course, these hormones, especially IGF-1, are intimately connected with metabolism, which is probably the set of pathways most heavily implicated in aging. So I am not sure this "coincidence" of aging starting when growth stops has a known explanation. It would however make an intriguing sci-fi novel if the "cure" for aging involved always growing larger and larger in size.