Gf kissed someone younger

I’m just gonna keep the shortened as possible, but basically I’ve been dating this girl for about five months and I found out a couple months ago ago that she kissed a 18-year-old girl that works at her job and my girlfriend is 30 years old. As far as I know, the girl has been to her house before I don’t know if she spent the night and I don’t know if they’ve hooked up but I do know that they kissed and I just don’t know how I feel about her being with someone that young, I’ve been trying to look past it but honestly, it eats me up inside because I think that’s a really big difference and I don’t wanna be with someone that is dating someone so young. She says the only reason she gave that attention is because she worked with her and she was lacking attention at the time, but I just don’t know if that’s a good enough reason for me to move on.