Well, if I recall correctly, in the TV series, Belldandy did tell Keiichi that they had a different perception of time in Asgard than they did on Earth.
But, also if I recall correctly, there were occasions where Urd and Skuld had to leave Earth and return to deal with a crisis, and their trips took no significant amount of time.
Also, from what I've read, it's never been explicitly said that the deity's in any ancient culture's mythology didn't age or die of old age -- that concept may have started with Judaism, and found itself intertwined with other cultures as time went on.
I honestly don't know, but I know this: it seems unfair to me that one of the closest couples in animé and manga will end with one half going on to the afterlife, and the other half living out an eternity without him.