Hm. I am an only child to my mother, but the middle child in the mixed family. However, I see myself as an only child, as that was how I was essentially raised. I never really thought about being the youngest or the oldest. Being an only child (for the most part) made me pretty self-reliant and creative. Since I lived in a very rural area growing up and usually had nobody to play with I had to come up with most of my activities on my own. I'm actually thankful for that as it's made me quite an independent person.
On the other hand, it did create some social anxiety and I don't have a very extended family network.