UTF-8 use with XML

Given the following element using a utf character (created by a user's
system): <BirthCity>Trenton?/BirthCity> I've been told my system should be
programmed to accept this.  I can't find any documentation which supports
yes or no to this premise.  Currently we reject this as not well-formed XML.
Please offer expertise concerning this issue.

Regards,

Craig Long
"Be Kind to Your Enemies, You Made Them."