Well, we're just in the process of thinking about it, and have decided to have just two for each twin - one of each sex. My best friend (female) will be one, primarily because I know that if something happened to us she'd swoop in and care for both children without blinking (and TBH she has the resources to do so), and to put it mildly, she's 'lived'... IYSWIM. The godfather for that child is a vicar, so we reckon that takes care of the faith side, there. The other twin, we're thinking of my DH's own godson, who is 21, because of the age thing - someone closer to their age, and the current evidence is that he's quite a good role model as well (you watch, NOW he'll go off the rails!!). The godmother for that twin is possibly also likely to be someone in her mid-late 20's, who is a GP, and has quite a strong faith.
So, I suppose the answer is that although strictly speaking, it's people with faith (and we are church-goers), having one eye on the child's general welfare, and people who could step in and also be appointed guardians should the worst happen to us, seems sensible (it's what my parents did - although for both my brother and I, we had an aunt/uncle who was both godparent and guardian).