As a child I chatted on chat rooms that mostly had adults, since there weren't many child-only chat rooms back then. I was probably a little older then your daughter, but not by much, then and I had some people who I would have potentially called friends, and it didn't seem to mess me up too much ;).
I'm personally a 32 year old man who volunteers with children regularly. I use to have 7 or 8 year olds recognize me and run up to play with me when I went to places like my church or the school where I mentored. We would talk and play all the time, and I don't think the kids were harmed by it. I did it because I liked kids, didn't mind an excuse to play like a big kid myself on occasions, and yes liked to help children teach children positive behaviors and messages (mostly the kids new me as a Sunday school teacher or as a mentor depending on context).
In addition I spend plenty of time online, and likely have made friends with young children, not that I always new the age of people online or remember all of them. I know when I played EverQuest one of the people I played with was a 14-15 year old girl, so not much older then your daughter, and while I didn't play with her as much as some others we got along well and did talk. In addition as a side effect of my volunteering and education I provide online, which is geared to adults, I have been contacted by older teenagers as well. Usually these discussions are short, my answering a specific question or two and being done, but I have had situations where a teenage contacted me that was clearly headed towards a bad decision where I took a little longer to speak with them, developing a bit of a friendship to prove I'm not one of those adults that 'just don't understand' and that they could listen to me, in order to try to guide them towards safer decision when necessary (I had no means to contact parents about my concerns since I didn't know their real identities).
I don't recall specifically having been in the situation of having regular contact with someone quite as young as your daughter solely through the internet, but I don't think it would be harmful if I was. Most people are decent folks, and some people may simply be friendly or enjoy a child's conversations, from simple friendship to common interest to even wanting to offer their knowledge and years of experience to help educate/mentor someone younger. I don't think you would have a problem with your daughter talking to an adult in person often, does the internet really change that?
And if you have an issue with your daughter having a friend who is older, well generally I don't have a problem with that so long as I know the original cause of the friendship and some context (I would be more cautious with an adult friend in person then online, but still not necessarily concerned). The whole Stranger Danger concept was drastically blown out of proportion by news sharing the stories about extremely rare abductions whenever they happen, making people think of them as more common then they are because we hear of them on the news and forget how many hundreds of thousand of normal interactions with strangers are happening every day that the news doesn't report because it's not interesting. Some reasonable discussion over friends is needed, but there is no reason to presume negative motives when 99.9% of the population wouldn't have them!
To be frank I think that there is an unjust fear of the internet due to the whole "new media is evil' trope. There are creeps out there, my volunteer work means I run into them far more often then not so I'm quite aware of them, but they are by no means the norm. The vast majority of folks on it are normal, just as in real life, and frankly a crazy person can do far less harm as a friend on the internet then in person so as far as I'm concerned it's safer to have a friend online in that regard. I don't think there is any reason to suspect a relationship just because it's online, and I definitely don't have issues with a an adult sometimes befriending someone younger since I use to do that regularly.
If your daughter has given you no reason to suspect anything wrong with the relationship, and you also trust her to be intelligent and not be talked into anything stupid, then let her keep doing what she is doing. I frankly think a 20 year old women online is less likely to drag your daughter into trouble then a peer her own age would do in person (teens talk teens into doing really stupid things all the time).
Furthermore, your daughter is at the point where independence is important. If you try to control her life too much now she will rebel against it. There are times you still need to as a parent, but it's important to pick and choose your battles. Frankly even if you had reason to slightly dislike the women she is friends with it's almost certainly not worth the battle to separate your daughter from her friend. By limiting when you take charge now you ensure your daughter is more likely to listen and respect you when you do, if you try to control her over every friend she makes that may not be ideal she will simply stop telling you about her friends and you won't have a way to warn her to avoid the really problematic ones!
Having said that It's good to be involved in your child's life, and know what is going on with her. I would still ask questions about everything, including her friend, how they met, and what they talk about. I'm not suggesting an interrogation, but simple casual conversation. It's good to stay up to date, and if through that you learn of an actual reason to be concerned about this friend, something she said or did rather then just her age, then obviously you might need to do something about the interaction. My main point is simply that there is no reason to presume anything is wrong by default, that of course doesn't remove the general job of any parent to stay informed about your daughter, her friends, and what she is getting up to with her time.