Apple IIe: Age 12
Macintosh Classic LC: Age 19
Performa 61xx CD: Age 24
G4 450: Age 28
2.8 24" iMac: Age 36
5 Apple Computers in 24 years, 4 Macs in 17 years, but I guess the frequency picked up a little. I kept my G4 forever though. Just sold it last month to a Linux guy who wants to mess around with OS X. It's still going.
][+ 1979
][GS 1986
Parents "switched" to PCs 1990 😱
(Got tired of driving their computer to my friend's house to fix it every few weeks, got myself a Mac.)
iMac DV 400mhz grape 1999
Cube 450mhz 2001
G5 iMac 20" 1.8ghz (refurb.) 2005 - current
MacBook CD 2.0 ghz (refurb.) 2006 - current
Note that my parents and sister switched back shortly after I got my iMac.
*If you want to stay on the leading edge, buy refurbs - you can literally get a new computer every year and sell what you have for damn near what you paid. 😉