Some parents are more comfortable with different types of dating at different ages.
Other parents do not care about the age of their children until "dating" becomes a one-on-one event.
Unfortunately, in these situations you have limited options unless you can determine that the person you dislike has actually broken the law or committed some other infraction according to an objective perspective. First, understand that informing your child of your feelings regarding their significant other is unlikely to be taken in a positive manner.
This is likely to be the case even if you are merely expressing concern as any loving parent would.
They may not tell you but their response may give you a little more insight into the situation.