As far as the age difference goes - for maturity, it depends on the individuals. A mature 17 year old and a nicely young 21 year old might get on fine. A normal 17 year old and a responsible, considerate 21 year old can get along fine, although there is a distinct power/experience imbalance. A 17 year old and an immature or selfish 21 year can be a total disaster - that's where the difference in experience and life situation makes a big difference. A 17 year old who is intent on dating 'up' (doesn't want a high school guy - wants one who can buy beer legally, and has more money to spend on her, and has his own place) can also set bad habits.
One important difference with a 21 year old dating a 17 year old is that the 17 year old is a minor, still under their parent's oversight. The 21 year old is an adult, who is responsible for their own romantic relationships without their parent's involvement - it's pretty common for university aged kids not to even tell their parents about their romantic partners/escapades unless it gets serious, and the parents can't demand that knowledge. So you can talk to your daughter about things, but you if there are problems then you can't go to his parents to address them, anymore than you'd go to your daughters in-laws to if she were having marital issues. She and he will need to work things out on their own.
On the more general topic - once a teen is of legal age, or graduated from high school, they are responsible for making their own romantic choices, and the parents' don't get a veto. The parents don't necessarily even get told what they are doing with whom. At this point, your best bet is having set the groundwork - raising a kid who respects themselves and knows that they deserve to be treated well, and who has the knowledge and tools to make responsible decisions in a relationship, (or fling), and to have a relationship with them where they can talk to you if they have concerns or need advice, and they'll respect your opinion enough to listen when you have genuine concerns. And to voice concerns judiciously - "I'm worried about the way he treats you" is very different than "I don't think he's well educated enough to deserve you."
For minors - the younger they are, the more oversight and control a parent can/should exert. 17 1/2 is almost to the point of letting go - 14 is a very different story. And there are times when a parent should intervene - abuse, risky behaviour, statutory rape issues - although that should be done in consultation with a psychological professional.
Objecting to a particular person is likely to drive them further into the relationship, even it it's not a good one, so a delicate touch is needed. Setting limits on behaviour in general, rather than a particular person, is easier- things like "No dating until 16" or "No sleepovers" or "We have to meet your BFs".