I'm more than 20 years older than some of my cousins, and they have always called me by my first name only. They are adults now anyway, so it would be silly to be called anything else. And I have one cousin around my age -- her son just calls me by my first name too. Why not?
Do you really not understand why not? I'm not saying that there is anything wrong with the way your family does it -- there definitely isn't. But are you truly unfamiliar with the practice other people have in showing respect and affection through using family titles?
As this string clearly demonstrates, there is a wide range of practice on this subject. Geography, ethnicity, family size, and many other factors play a part.
Many adults call adult relatives, especially older ones, by a family title -- like "Mom," "Grandpa," or "Aunt Clarice." (And other families even call grandparents by first name, as we have seen in this string). How is that "silly"?
For many people, "Cousin David" is really no different from "Aunt Clarice." For others, it sounds unfamiliar, and they don't do it. So what? I am perfectly fine with my northern cousins calling me just by my first name, but I'd find it very rude if they laughed at my southern cousins calling me Cousin Gell.
It isn't necessary for everyone else's way to be wrong for your way to be right. There is no need to declare that it is "silly" or "Victorian" or "strange" to do it the way your family doesn't.
I think this is an area particularly deserving of caution in this regard, as there seems to be a strong association of ethnicity and family title practices.