There is an unspoken limit on how much older one person can be that the other if they are in a relationship. While this unspoken limit does not mean anything, especially to the people who are "breaking" this limit, some people will judge other couple based on age even if it is wrong.
Yes, there is an unspoken limit on how much older one person can be than the other if they're in a relationship, at least, societally speaking. If a one partner is too much older than the other, the rest of society will view the relationship quite unfavorably, and this may lead to shunning.
Though society may dislike big age gap relationships, who cares if the couple is truely in love. I am in a 24 year age gap relationship and it works for us because (from what he says) I'm much more mature and wise for my age than other girls my age... I don't want to party or any of that and I hold more traditional values just as he does. But it all depends on the people, if there is not as many interests that are the same between them, because of one of them being younger, it may not work as well... But I don't believe there is any limit on an age gap between the 2 people in the relationship. Society may disapprove but ultimately it's the people in the relationship that can or can't make it work.