Maybe that's using the (false?) assumption that they're all Player Characters. 
This is a little like a thread I tossed out a while ago, trying to estimate how many "Spring Covenants" were generated each tribunal period.
Any one of the variables requires a best guess, but still one based on little or no hard data - multiplied together, that's a lot of guesswork goin' on.
Another way to approach it would be to take the total number of magi in the order (a number I believe we do have a figure for), and work backwards against a life expectancy with no accidents, just aging. Again, this would require a lot of fudging and faking, but if the two ended up close, that would be a bit more reassuring.
But if it gives you "an" answer, that at least feels better than a pure blind guess, even if, perhaps, not much more reliable.