With old deer, teeth are the biggest health concern, spoil them a bit more than others.If you've kept them till 15 they must have survived several culls,no doubt for a good reason.A fawn from an old girl is less likely to have a genetic health fault than a fawn from a young girl.This is the same reason I rarely allow a stag or buck to breed before 5,it allows any genetic faults to express ,which we can not see by eye.Generally speaking having some granny does in with the mob makes yarding & handling so much easier,as they calm down & lead the others around.I have several granny's who will still produce some of the best fawns,even if it's only every second year.These girls will continue to be spoilt as long as there healthy.Cheers Sharkey