No. Apple provides security updates for several versions. They most recently patched NTP for Mountain Lion, Mavericks and Yosemite at the same time.

http://support.apple.com/en-us/HT1222

Apple provides security updates back somewhere between 36 and 44 months for the OS. Unfortunately, you won't know until they announce some exploit and don't patch your OS if you've been left behind or not.