"Dealbreaker or acceptable? For me, it's not a dealbreaker, but as the age difference increases, it becomes more important how both people think and feel. I think an 8-10 year age gap is normal and can be handled, but anything over 12-15 requires a certain level of maturity.
Comfortable range? Like I said, a 5-10 year age gap is fine, but anything over that requires a lot of effort to handle.
Younger or older? I think a man should be older than a woman, but the opposite is also fine. It gives the man more responsibility and the woman more security.
A word of advice: Don't romanticize the age gap, if you have any issues, talk about the future. For example, when to get married, whether to have children, career priorities, pace of life. Many age-gap relationships fail because they don't understand the difference. "Ignore things."