You actually get 800km out of a pair of shoes?? That's incredible. I usually find that if I pass the 600km mark I'm tempting fate with shin splints and heel pain. North have you tried the NB900 yet?
I'm no expert on age/growth etc. As far as should you run in May...well that all depends on where your mileage and fitness level are at. Certainly if you're not in shape to run a marathon, you won't have any fun regardless of age.
BTW: we're mostly tame here although it does get a little tiresome to follow the tangent "debates" that some of the discussions take.