You make some good points there. At times, I find myself at a loss for words, and W has interpreted this in the past as me being silently judgmental. I just have a hard time with certain things that she presents.
For example, in March I mentioned that as soon as I could teach S5 to swim, I'd start teaching him how to surf. W goes out once in her life, and notes that her 24 yo GF/student would teach him. Just occasional cuffs to the nose that sting with the message "I am with her" (not sure how yet, likely just having a pursuit of youth, as UD says), and "We'll have fun with S5 without you."
There is definitely a competition thing going on, but not from my end. I'm confident in what I can do and offer, and I know that I'm not replaceable in S5's life. The reality is that this young friend will be out of her and S5's life very soon, and its merely a wish on W's part.
This 24 yo is not as wise as your 14 yo acquaintance. She reminds me A LOT of W at that age - impulsive, overly assertive, and full of confidence - all things that W admires, and maybe wants to recapture.
From a positive angle, perhaps she is trying to connect to me, or merely display some initiative?