Well legally, it's illegal in many states. Although the technical age of consent is below 18 in most states, there's usually a provision that if the girl (or guy) is below 18, then the other actor must be no more than 2-3 years older. So a 16 and 19 year old is okay, but a 16 and 21 year old is illegal. Google 'Close-in-age exceptions' for more information. Again, this is the legal position of some/most US States, but not all.

Morally, I would say that due to the gap of life experience and maturity, most of the time that is an unacceptable age gap. 10 years doesn't mean much between say, a 45 and 55 year old, but 10 years is literally over half of the 17 year old's life. Sure, there are incredibly rare, special cases where the 17 year old is truly mature enough to date a 27 year old and both are entirely satisfied (beyond mere sex) with the relationship.

In my opinion though, the majority of 27 year olds in that position are predatory, because most 17 year olds are merely seeking validation and attention without understanding that sex is not a substitute for either.

When I was 23, I dated a 19 year old and I felt a little weird about that. She couldn't come to the bars with me, I was done college and she was just starting, etc. She was very mature though, and although we did break up, age was not the reason (it became a long distance thing since I was trying to start a career, and she was still in school). I think in general, age matters until you hit your late 20s.

Hopefully I qualified this comment enough. Again, I'm not saying there are NEVER any circumstances where this could work, so everyone who has one ancedote, please save it. I'm saying that usually, this is a major red flag.