We are matching now for an August arrival. Last year, I started in late February, and it took me until early April to find the right AP. March flew by with all of the good applications, which I was just starting to view, disappearing by the day. I wasted enormous amounts of time writing introductory letters and those first get-to-know-you emails to APs, only to have them match with other families because they were further along and the APs wanted a family (and often were saying yes to the first family who asked). I also thought I was looking for something different - an Educare (APIA) - than we ended up matching with (a regular AP), so this added to the time it took and why it took me so darn long. Anyway, I vowed after last year to start earlier this year. We've been screening applications for three weeks now and interviewing two candidates for the past two weeks. It takes me a couple of weeks to interview and do my due diligence, and I like that there aren't too many families competing for the APs' time right now. Our matching coordinator, or whatever she is called, told me that while they will add some summer applications between now and end of March, by and large most of them are there already. And the go-getters and proactive folks have been there for months already, so why not start reviewing them right away?
The only downside I can see to matching so early is that it's then a long time to sustain a relationship with the incoming AP while at the same time needing your time and energy for your current AP, but hopefully you can get on a good schedule of a weekly email and a bi-weekly (short) skype call, so everyone feels connected but it doesn't take up too much time.
About extending: we match with 19 yr old Germans who return to go to University after one year, so our APs never extend, but I think most agencies start talking about it four months out. There is LOTS of info on aupairmom.com about extension APs....you might want to read a few recent threads about extension APs before you decide to go down this path (not that it will convince you not to, but you will go into the process with eyes wide open).
Good luck.