I think the best way to wear them is like Diana posted in her celebrity picks: with just a sliver of midriff exposed. If you expose the belly button, that I think is where the look starts to look more youthful than you may want.
If the crop top is a little longer, you can sometimes create outfits where the skin is not exposed at all while you are standing still, but when you move around (put arms up, bend over), that is when some skin is exposed. I think Angie has mentioned this in her blog in the past, but I don't know where. Maybe do a search on her blog for "crop tops" for more ideas?
If you look at the Link Love for this week, there is a link to fashion bloggers' husbands' comments about the bloggers' outfits. A few of the outfits showed the sliver of midriff with a crop top.
When I wear crop tops myself, I try to pair them with high-waisted bottoms so that not too much stomach shows.