I think it's poor form to ask out a female employee who works at a coffee shop that you frequent. When women are doing their job (which involves being friendly to customers), they usually do not want to be hit on. Being hit on at work is a common pet peeve for many women. It's pretty disrespectful to hit on a woman who is there to do her job and provide you with professional service. Especially if there is a significant age difference, it could make her feel very uncomfortable. If you like having casual conversations with her and going to this coffee shop, I would recommend not asking her out. If you ask her out and she is not interested, it could become very awkward. She might avoid you or have someone else wait on you after that.