How old is Zemnas in Kingdom Hearts 2

Xemnas, along with all the other founding Organization members, is not very old, somewhere in the teens, if one looks at things literally.
This is not to say that Xehanort isn't older, as Xemnas was created when Xehanort was an adult, and therefore, Xemnas is only as old as he was from the time of his creation.
NOTE: This does not apply to:
Xehanort's Heartless
Ansem (Xehanort's Alias)
See also:
Braig
Ienzo
Even