@turquoise-- I don't know if there is an age group that most often experiences hysterical pregnancy, but I know that there is for pseudocyesis. Pseudocyesis is basically the same thing as hysterical pregnancy except that the woman continues to believe that she's pregnant despite medical testing that proves she is not.
It's not a very common condition, but it's mainly seen among women in their late 30s and early 40s. Basically as women get closer to menopause and especially if they've never had children despite trying.
Just like hysterical pregnancy, these women also show false symptoms of pregnancy like nausea, weight gain and so forth.