I'm an only child who's 44 now, and yes, I wish I had a sibling. My situation was complicated by the fact that my mother suffered from manic-depression (they call it bipolar now) for most of her life, and my father died in a car accident when I was 9. Through certain turns of events, I actually turned out happy and relatively well adjusted.
As I get older, though, I really wish that I had "family" to relate to and share things with. My mom passed away when I was 30, so I'm the last one. My husband's family is wonderful to me, and I really appreciate that.
I'm happily married, and I have 2 kids. I always knew that I wanted more than one given what I felt I missed out on when I grew up.