I know right now some people are going to have issues with what I'm going to type, but please read through this whole post. I'll try to keep it as simple as possible but hold through to the end.
Is 9 to young? Yes and no.
I've been fortunate in my 30 years on this planet to have traveled been exposed to some cultures, and having lived in other countries for extended periods of time, I want to share so world views I have.
First from a gamer side. I do not think it is a good idea to put a nine year old with older people playing an MMO (if supervised this doesn't apply) un-supervised. There are as some have stated very colourful people on the internet that will not tolerate a 9 year old. They won't have the patience or the decency to conduct themselves in a manner suitable for a child. I'm guilty of this and I challenge anyone else to say they wouldn't be guilty as well. On the internet people tend to disassociate themselves with society as such we become someone very different we aren't always conscience of who or what sits on the receiving end of our typing. But I do know tensions could be flared if someone who may be young doesn't understand the finer points of party game play in a group. This is where I would see you having to place a limit on the child'ss interaction with others. Otherwise the rest of the game has been put to a level that I'm sure a 9 year old could grasp just fine.
As a person I think it is important that you first understand what the child really knows. I have found in my travels that children in Scandinavian countries grow up and mature mentally faster than North Americans. This is because of one thing, censorship. Censorship is a hell of a lot more lax there. As such radio and television don't beep out swear words (at least on channels I watched), it isn't uncommon to find nudity (full!) in adverts, in a bus shelter, downtown, at the main station. North Americans live incredibly sheltered lives in comparison to our European roots, and I must say from a personal stand point I found living there much more liberating and less stressful than in North America. I also found that if a person could raise a child in this atmosphere certain other issues could be avoided in a child's development if the parents were proactive enough in the up bringing. That being said I did run into some like bastards that could have used a good smacking. When it comes down to it, it is all based off how much you are going to put into the child's development.
My point here is that if you are going to let the child play be sure your relationship with that child is good enough that you and the child can talk about their experiences without the child feeling ashamed of something they have read. The child will learn what peoples real colours are like. You also have to be sure the child is of a certain level of maturity as well, other wise this will back fire tremendously. Again it is one of those cases where yes you should be supervising them. But exposing a child to these things aren't always a bad thing as long as you are teaching them sound moral rights. I can tell you will almost 100% certainty that this child has likely seen heard and read worse, unless of course this child is home schooled and lives in a plastic bubble where their only access to the outside world is their own yard where they have only seen another child that has been approved by their parents. If the second is the case I'm sorry to say but the child will likely be scared for life if you drop them into this community regardless of supervision, you are better of finding hello kitty adventure island for them.
By the time I was about 7 or 8 I knew just about every swear word imaginable and some in different langauges. I learned these at school on the field from older children. Parents need to remember back to when they were kids and things that they really learned when mom and dad weren't looking or around. I knew as a kid growing up these words weren't nice, but I didn't know why or the meanings behind the words (this landed me in trouble more than once). My word of advice is to talk to the child, learn what they have learned and explain that they wouldn't be in trouble for using them in your conversation. Ask them where they learned the words and how they were being used. Ask the child if they know what the words means and why society considers it bad. Teach them what the words mean and why they are bad, don't wait for the child to be in trouble for saying something they don't even understand because then it will be a lot more difficult. I think the parents that raised children in the 90s and 00's but even some from the 80's (I was born in the early 80's) as well dropped the ball. Video games and television became the new supervisor without parents really understanding what their child was getting out of it. If this child has used the internet unsupervised I'm more than willing to bet they have seen and read things that may or may not shock you. Children aren't as innocent as they seem, where parents need to step up is approaching these children and helping them understand what they are experiencing. Do not shelter the child it is important they understand what the world really is and that the parents aren't always going to be around and that they need to understand that people can be mean. What you need to do is how to deal with those emotions and how they should deal with what they confronted with.
I'm going to end it here before I get lost in big picture. But my hat off to you though for approaching this issue very sensibly. I can only hope you do a better job raising the child than some of those that now populate this world that we share.