We hosted a German boy for about 3 weeks, when my son was a junior in high school. Lars was maybe a year older at most, so they got along fine. My son had had 2 years of German, and Lars’ english was very good, so communicating wasn’t much of a problem. The next year, my son went to Germany for 3 weeks, and stayed with Lars’ family, so he had some idea of what to expect. A year or 2 after, Lars and his brother came back to the US on vacation, and stayed with us for a week or so. We were basically a free place to stay that time, but it was still nice to see him.
I 2nd the idea of going with a short-term stay if this is your first time. Our high school has been doing this sort of exchange for about 20 years now with a school in Germany. We’d send 20 or so kids there in the summer, they’d send a passel over when their schools were on break. Each group got a chance to go to school with their host family, but they also did visits around the country. Lars and his group went to the state capitol, visited a large american university, went to Chicago shopping, etc. We took him with us on a weekend trip up north to Mackinac Island, too. The exchange kids and the host kids did a lot together as a group - sports, shopping, etc. They LOVED the mall - there was nothing like this at home in their town.
There are things you need to work out ahead of time. If their culture or legal structure treats an issue differently than yours, you need to be clear on it up front. Smoking was mentioned already, if that’s a problem for you. Drinking is another. Lars, coming from Germany, was old enough to drink legally at home. Here, we didn’t let him. Also, driving, curfew, music, dating (or overnight guests) and religion come to mind. Phone usage, too - if your exchange student needs to call home every other day, you might want to be sure that both parties can use Skype ahead of time. In fact, that might be a good way to communicate with them NOW, before they arrive.
I’m not saying don’t do it. If I had another kid at this age, I’d be happy to do the same thing all over again. I only want to point out areas that you can and should address ahead of time.