Alright, D-Man. I see you're a fairly respected guy around here, i'll give this a shot.
What do you call an acceptable age difference in a relationship?
This is kind of a loaded question and it's tough to give a good answer. Unfortunately, we live in a world where slight differences in the situation can affect the answer to this question.
On one hand, it's not acceptable in our modern day and age to date a 15-year old if you're 25 years old. And yet it's perfectly fine for a 35-year old to date a 25-year old. Now, one might say that it's because the 15-year old is underage and that would be considered statutory rape... understandable. But then why is it frowned upon for a 34-year old to date an 18-year old? Here lies the problem...
In my opinion, it all comes down to abiding by the laws that are given to us and staying within the boundaries of maturity. In other words, first and foremost, if you're old than 21-years old, there is no reason why you should be dating a boy or girl that is younger than 18-years old. (I accept the fact that 18-20 year olds tend to date 15-18 year olds at times and I don't believe this would ever be considered statutory rape... but that's up to their parents.)
But let's go into the equation that both parties are over 18 years old and by American laws and standards, it's perfectly legal for them to date. If that is the case, you need to view the younger party's maturity level. I believe it is acceptable for two people to date, regardless of age, if both parties are handling the relationship with an equal amount of effort and maturity. If two people are attracted to each other and build strong feelings for the other, I don't see anything wrong with them trying to make things work as long as both of them are willing to accept the consequences that come along with the age difference. While a 50-year old dating a 22-year old might seems disgusting to many of us, who are we to judge? As long as they're both willing to accept the scrutiny they are going to face from judgmental people and are willing to take the risk of living two completely different lives of maturity, I see no reason why they shouldn't date.
I hope this answers your question.