The response to that is "why not" and then go keel her over and start making out with her. Trust me any girl that is sending you signals is extremely doable, and extremely #$#@$$$able. It does not matter how young or how old they are. You literally could put your hands all her body and make out with her on the spot, even do her right there and then. If she is your kid sisters friend, your professor, your school teacher, your physician/doctor or whatever secretary.
It does not matter what age she is. You could put a smile on her face you can get with her.
That could mean many things. Usually it is because she made choices in life she regrets and because of those choices her method of thinking is wrong.
Women evolve and changes while boys are boys for life. The only difference is that we eat their terrible food, work like dogs, and deal with their lame family members all for the need of company.
First she is a girl
Then she gets her period and becomes a woman
Traditionally gets married and becomes his psychiatrist
She gets pregnant and becomes a mother
If she has a son she becomes his idea of his wife/girl/woman/etc
If she has a daughter she has somebody to coach in life and be a friend
When they leave ( if they leave ) she can move on to other business.
Depending on how the relationship is, or income is things could vary.
She could lose the ability to have children ( which could happen at 25-??? ). Bare in mind many women have children around 15-20-25 as a way to have more kids more easier.
So forth and so forth and so forth........
It is not that hard to walk up to a girl, ( anybody ) tell her how you feel and then invite her back to your bedroom, and "get with her". Trust me they are predators and is built into them.
Another factor about "the ladies" is that any girl who is doing well at any moment is far more able to sleep with if she is doing well. Financially, good-looks on the outside, and more.