Asclepiads and Frailea seem to go "off" very quickly, and I believe seed of Pediocactus, Toumeya and Sclerocactus need to be as fresh as possible to get good germination, but I think generally things last longer than is commonly thought. I just sowed a heap of 2.5 year old Copiapoa seed and it came up like absolute cress!
I've brought Mammillaria theresae and luethyi up from seed, and had reasonable success with the Mexican miniatures like Aztekium, Arios, Blossfeldia and Geohintonia but one group I struggle totally with is the Mammillaria saboe-haudeana-goldii complex. I have tried seed so many times of these but always with zero germination. Anyone had any success with these??
The Opuntias are also a pain in the you know what ! So hard, but I think they are more about giving them the right conditions and pre-treatment rather than age...