WindowsXP is based on NT which came from the work MS did for IBM on OS2 (late 80's IIRC).
MacOSX is basically a flavour of UNIX with a 'Mac GUI' over the top. The last of the 'Mac based' MacOS was MacOS9.x. If you want to be pedantic UNIX has its roots in the late 1960's.
The age of an operating system does not necessarily define how well structured it is or what features it provides. Don't forget that the Sinclair QL had a multitasking OS in the mid 80's.
Perhaps useful to remember that the OS and the GUI are separate things (on a functional level although sometimes difficult to seperate on a technical one) and should not be confused, particularly in such discussions as here.