hmm.. my view is, "Mdm" sounds old only if it is followed by the person's name, eg. Mdm Tan, Mdm Wendy Lim, etc, and it is usually used to address married woman in a old-fashioned way .. and most woman will be offended if addressed as such, esp. young married women.
Whereas addressing a lady as "Madam" by itself without following with her name, sounds polite, esp. addresssed by a service staff ..