Regionalism does play a part. In the South and parts of the Southwest, many people of varying ages would say "the lady", or call her "ma'am," even if she were considerably younger. The exception would be if they were being casual or cutesy, where a woman might call another woman a "girl" or "gal" in a friendly manner. It took me the better part of ten years to get used to that habit. I grew up in the Midwest, where we would use the term "woman", and my experience in places like California, Boston, New York, etc., is that it's common currency there too.
You may also refer to her as "The person who I spoke to" - independent of gender. If you continued speaking you'd probably refer to either she or he (e.g. "The person I spoke to was very patient with me. She told me where I could go to find the right paperwork I needed.")
Someone reiterated that you should not call a grown woman a "girl" in any professional context. This is true, unless you have an established friendship in a casual setting where she has already used similar language (for instance, an off-the-clock meeting with a group of colleagues at the bar). I would say overall, avoid.
Likewise, never make assumptions about a woman's marriage status, and avoid using terms that designate marriage status for a stranger, unless she tells you specifically to use it. It is always OK to call a woman in business or professional settings, "Ms." (Pronounced "Mizz") because it can be used regardless of whether she is married, or not. It's been used for hundreds of years but became more recently popular. "Miss" and "Mrs." designate marriage status.
About ten to fifteen percent of women in the US keep their name after marriage, as I did, which is another reason why Ms. is a better catch-all title. "Ms. Tail-Kinker" has been correct my entire life. It is incorrect when people call me "Mrs. Tail-Kinker", though, even though I am married. Tail-Kinker is my surname or "maiden" name, the one I got from my father's side of the family. In Spanish speaking cultures I believe it is typical for a woman to be called "Mrs. Tail-Kinker" after she marries, rather than "Mrs. [Husband's name]". But in US practice, the term Mrs. refers to a woman using her husband's name, not her maiden name. If you therefore call a woman who has kept her original name "Mrs. Original Name", she might be offended or annoyed. Many of us particularly find it extremely rude to refer to a married woman as "Mrs. Husband's First Name - Husband's Last Name".
Again, the safe bet professionally is to call someone "Ms." and then follow whatever they ask you to do specifically.
