Good point, Godot, but the reason I thought they might be younger is because, in my own experiences, when a guest list jumps from 150 to 250 it is usually a younger couple who all of a sudden have discovered protocol for invitations to weddings, or who have family members (like the parents or siblings) who have suddenly mentioned that if they invite so-and-so they really ought to invite this person and that person. Generally, in my own experiences, the older the couple the less likely the numbers change too much from the original count, which has usually been padded to some degree anyway.
At a wedding we attended a while ago, the couple had no idea of invitation protocol within even the family structure or boyfriend / girlfriend / significant other or partner relationships etc. When they discovered their mistake the guest list changed really quickly. Generally speaking, older couples already seem aware of the social graces, where often times the young folks Re still learning.