I'm going to refuse to guess coworkers age while in a professional work environment
Why do you not want to guess?
I messed up once and refuse to do it again.
If you are uncomfortable answering it, then don't. I am sure your coworker will understand the situation that it will put you in.
Yeah I will let her know.
It is good that you are letting her know. Everyone has different boundaries, and as coworkers, you need to be respectful of each other.