Some of the same factors that trigger the growth of antlers also trigger the shedding of those antlers. To keep it simple, a decrease in testosterone levels trigger a weakening or eroding of the pedicle base of the antler. Once it has weakened or eroded to a certain point, the antler will drop. Because the growth and subsequent loss of those antlers are triggered by testosterone levels, younger bulls and bulls that are past their prime (If they live long enough) can and will retain their antlers longer than a prime bull. They do not yet have, or have lost, those strong "triggers". Of course, it's mother nature we are talking about so there are always exceptions. Weather, stress, and many other factors can and do have effects on a bull's testosterone "trigger" cycle also.
As for when they are shedding, they should be doing it now, or VERY CLOSE to now in most areas. In fact, I believe another member just posted some fresh Rosie sheds a couple days ago.
And, as for growing them...the pedicle will heal over after the antler is shed. Soon afterwards, you will start to see the tell-tale nubs of new antlers as they grow their new set of headgear. Again, the rate is dictated in a large way by the health of the bull, his age, and his stage in life.
Lastly, you asked at what age do bulls shed their first rack. Well, unless the bull has issues with his testosterone levels (Injury, castration, etc) they shed their first rack after that first year's cycle. In other words, they shed their ranks at relatively the same time of year whether it's their first set of bone or 8th set of bone (With all other factors above taken into consideration)
I am not a biologist so if someone has something better or can point out where I might be wrong, please do so. I just study this sorta stuff as a hobby.
