I'm both an only child and a youngest child (so much younger that I was raised by myself; I am my parents' only child together; and they always referred to me as an only child). I identify much more with being an only than being the youngest. My relationship with my siblings really only developed as I became an adult. I relate to some traits mentioned in this thread, but not all of them.
One thing I will say is that I will never again be in a relationship with a middle child.
My two longest and most important relationships were/are with middles, including my husband. I hate the neediness. If I marry again, my next husband will be an oldest.