Believe me it's not all it's cracked up to be, some foals just hit the ground with attitude.
I've had colts here that I've barely touched before weaning, and they train just as nicely as if they had been handled from day 1, there is really no magic it in. You really can't do to much with really little ones anyway, they are learning to be a horse, and they don't have the attention span to do more than a few minutes every day.
I have some sympathy with someone who wants to breed a mare that they have had for a long time, I just struggle with buying a mare just to breed her and have a baby.
I do have to admit though that I bought Ace mainly because of the foal that she was supposed to be carrying. I got a chance to buy a mare, who I liked, in foal to the stallion that I've wanted a foal from for a long time, guess what, she lost that foal. I've been offered a chance to breed her back but I've retired her from breeding, she is 18 just had her 7th foal, and enough is enough, much as she enjoys it, it takes so much out of her, so for Ace, 18 is to old.
My friend is waiting for her 23 old mare to foal, her vet was happy for her to rebreed, so who knows how old is to old?
Apart from age there is also the question of being suitable mare to reproduce.