[btilly]

If you make the naive assumption that you are observing the company at a random point in its overall lifespan, the median of the distribution for how much longer it will survive is how long it has survived already.

More than that, if it has survived N years already, the odds that it will survive at least M more is N/(N+M). If you work out the probability density and then integrate it, you come up with an arithmetic mean that is also N. However the point where it is most likely a priori to die is tomorrow, and that probability steadily decreases.