a) How many colleges you are interviewed by depends on the subject you apply for - for some, everyone's interviewed by at least two colleges.
b) again this is subject dependent but no, you may not be interviewed by your chosen college at all if your subject equalises the applications before interview, in that case you may be reassigned to a completely different college. Nothing to do with age except that obviously an under 21 couldn't be reassigned to Harris Manchester.
c) No, not except as it applies to you in a) and b).
Basically college applications are the same at Oxford for over 21s as they are for everyone else, except that you have extra options.