Apple has gone to great lengths to make recent versions of OS X, including Mavericks and Yosemite, available on a wide range of Macs dating back in some cases to 2007.
Based on information provided to early testers of the first developer beta, OS X El Capitan system requirements for the developer beta are the same as those of the current public version of OS X, Yosemite. Specifically, OS X El Capitan lists compatibility with the following Mac models:
iMac (Mid-2007 or newer)
MacBook Air (Late 2008 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
Mac mini (Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
Apple boasts variety of performance enhancements which will be accessible in OS X 10.11 El Capitan. However, it is unknown at this time if certain Mac models lack the hardware to implement these new technologies, and will therefore be limited to some of the OS X El Capitan features.
These are the system requirements of the developer beta only. Apple may change the requirements for the public launch of OS X 10.11 El Capitan, although the company has not changed system requirements during previous OS X beta programs in recent years.
OS X 10.11 El Capitan will launch this fall via the Mac App Store, and will be a free download for all users with compatible Macs. As with OS X Yosemite last year, a free public beta program will allow non-developers to preview the operating system starting in July.