ma'am - short for madame or mademoiselle. both french if my memory serves me correctly. just as in spanish with senora and senorita (too lazy to add the tildes), it's simply the difference in marital status that seperates them.
I answer ladies of all ages as ma'am at different times. just something i was raised to do. if i'm going to say ma'am to a 18 year old, then there's absolutely nothing wrong with saying it to a 29 year old (and the manners my father whipped into me prevent me from believing that any of you ladies are a day past 29
). the problem is that people these days (especially outside the God-blessed South) think that ma'am should only be used in reference to an elderly lady. I see it and utilize it as a term of respect for ladies of all ages.
holding doors is something i do all the time, and thankfully, here at Texas A&M, there are quite literally thousands of people who do the same--moreso for ladies than for men, but most of us just hold the door period as a matter of courtesy.
personally, if i'm out at a restaurant on a date or in a group, I'll stand up as a lady leaves or returns to the table, but don't do it at a McDonald's or in the dining halls. more of a enviorenmentally-appropriate usage of that courtesy, and usually done when i'm sitting in a chair and not a booth simply because booths make many things impractical.