IMO, I don't think the age of a bitch has anything to do with how good a mother they are.
Some bitches are just naturals, and some just don't adapt to motherhood, don't really know why.
My bitch who had 2 pups 8 weeks ago, is 4 years old, and she loves the pups to bits.She plays with them, and still would let them suckle her if we allowed it.She is a natural,although I don't know what she would have been like if she'd had a bigger litter