does it have a drawing of the white house on the front or maybe it was a painting of the white house ? I ask because my parents also got one and at that time I sorta assumed that they got it because they voted for him or because they were registered democrats. They would have been in their 50s when they got it so I doubt that it had to do with their age.

The only other reason that I can think of for them getting the card is that they would have been married over 25 years so maybe Carter was sending them out to those who had lasted through 25+ years of marriage.