Good crema comes from fresh beans. And fresh doesn't mean 'I opened the tin yesterday' ... think of coffee as like bread - if I bake a loaf of bread today, put it in a plastic bag (even if I nitrogen flush it, suck all the air out of it, treat it with whatever chemicals) and then open that bag in a few months time, the loaf of bread will not be fresh. Coffee is the same - it doesn't matter when the bag/tin/whatever was opened, it's all about when it was roasted. Depending on the coffee, most beans are at their best anytime between a few days to a week or two after roasting. After that, you'll struggle to get crema, but even worse, you'll struggle to brew a shot that is actually enjoyable to drink.
