Hi and welcome to the board.
I have a 19.5 yo dd and if she met a man who was 28, the age isn't what I'd be looking at. If he was lazy, aimless, had no interests outside of her, working at a dead end job and not doing anything about it; if he was controlling (sometimes someone much older wants someone much younger because they are easier to control) or manipulative, a liar, drug user, drank too much - then I'd be objecting. All things I'd worry about at any age, but these are the specific kinds of things I'd look at with an age gap.
If, however, he was a decent young man, held a good job, seemed 'settled', treated my dd well and with respect, I would not raise issues. I'd likely point out that over time they are going to notice the age differences if they are in two very different places in their lives. However, my dd has always seemed mature for her age and adults or people several years older than her have always felt comfortable around her and enjoyed her company. She doesn't chit chat about silly things; she has goals and loves to travel; she is financially savvy beyond her years. Even when she was 14, and went with our church youth group out east for 10 days for a youth convention, our group consisted of one kid her age, the youth leader, a guy 21 and another guy, 19. The two who were 19 and 21 both preferred to team up with my dd over even each other. They liked her as a *person*. If she were to break up with her long-time bf (7 mos older than her) and met someone who was 28, at this stage, I'd trust her choices. She has a level head and knows what she wants and what she doesn't want. My closest friend married a man who was 28 when she was 19; just be glad your dd isn't talking *marriage* right now, lol. My friend's marriage didn't last but it was because HE changed dramatically; not her.
So, if your dd is basically level headed, and the guy finds something in common with her, and respects her and is worthy of respect himself (think: what would you think of him if he wasn't possibly itnerested in your daughter? - it'll help to separate the issues), don't let the *age* cause a problem between the two of you. Evaluate all of what is involved before you decide how to handle this.
Now back to your original question....at 19 they're adults and have to make their own way, as much as it hurts us as parents sometimes. (That's a lesson I'm still learning, as you'll see if you go over my posts from the past 4-5 months!) Even when your DD is living in your house (as my 19 y/os do), what she chooses to do away from home is essentially her business. We do maintian house rules for our older boys, i.e., you're not letting your g/f sleep over in your room, if you have guests over after 1030 on weeknights or 1130 on weekends you have to keep it quiet coz the rest of the household has to sleep, pick up after yourself, etc. But as far as what they do away from home, they're adults and have to answer for themselves. As far as the age difference with your DD and the man, I agree with kkiana...if he's a good man that treats your DD with kindess and respect, the age difference may not be an issue. If he's no good, he'd still be no good if he were 20 or 21. One of my employees is 21 and living with a man who's 30...he's had a very stabilizing and maturing influence on her, she's become much more responsible since she started that relationship at 19...but he's also a good, kind, hardworking man who treats her with a lot of love and respect. When I have concerns (OK, heartaches!) over something my 19 y/os are doing, I find that I have the most success dealing with them by sitting down and having an adult conversation about my concerns. And I work very hard to keep it a two sided conversation, because they are adults and have the right to make their own decisions as well as have their own opinions. With S especially, because he was raised in such a different household, I often tell him that if I didn't love him so much this issue wouldn't cause me so much concern, and I encourage him to just think about what I've had to say and take it into consideration when he's choosing his course of action. There is only a couple of things that we have hard and fast rules about for the older boys: 1. NO drinking and driving, no questions asked. Either stay where you are or call us for a ride. 2. If you aren't coming home at night, CALL because mom worries alot about things like car accidents. 3. If you're having problems with something, please don't wait until you're in over your head to ask for help - we're more than happy to provide help and guidance but it's so much easier if things aren't completely out of hand when you ask for help.I hope this has been helpful. Let us know how things go.