You have to go "No Contact." Thank her but tell her not to contact you again because you don't work together anymore.
Several years ago I was in my late 40s and an extremely smart, very high achieving woman about age 25 in my office started hitting on me. I was flattered like crazy. Later, MANY women in that office, some as young as 22, started flirting with me - almost like they saw her and then mimicked her. I was a hot commodity and it messed with my mind.
It was ALL very very toxic.
Eventually I asked her to meet me at a restaurant outside the office so I could figure out what she was thinking. She stood me up. Then she acted like I asked her out, which I did not. Then she acted weirder and weirder to me. Then I found out she told a coworker she imagined I would be the "Future father of her children," which this coworker didn't recognize as profoundly weird.
Then all the women who were flirting with me stopped flirting on the exact same day. Clearly someone said something.
It's a roundabout story to tell you that: 1. Younger women know they can easily attract an older man
Very likely nothing will happen because it's a mind game
When a younger woman hits on an older man, it's usually evidence of childhood abuse of some kind (Absent father at best, every other possibility- the worst), and you don't want to go there.
Block her. This has to end. This will mess you up worse than you can imagine.