I'm a 39 yo only - DH and I have 3 children. DD is 12, and DS's are 10. I always wished I had siblings growing up. Now that I'm older and my parents are getting up in years and beginning to experience failing health, I truly wish I had siblings to talk with. I have 2 great SILs that I count as sisters, but they're several hours away and can't help me with the parent situation. I have made a huge attempt in raising my 3 kids to make sure they understand that taking care of family and each other should always come first in their lives. They bicker, but do not have all out fights. This may seem harsh, but sometimes I remind them that when they are grown and I am gone, all they will have is each other. Treat each other with respect. All three are close and love each other dearly. Oldest twin is very much independent while youngest twin is happiest in a group situation. DD wants a sister, but that isn't going to happen

God gave me twin boys - and I'm done.
On another note, I work in human resources. I can usually tell you who the only children are in the office environment. There are a few that surprise me, but usually, the "only child" employees are a little less giving in everyday give and take work situations. This is not always true, however I have seen it on many occasions. And I'm sure I was less "giving" in many situations when I was younger and had no children. It was all about me. For this reason, I strive to teach my children acceptance, and a happy medium to difficult situations. Don't be selfish, put yourself in that person's place.
Being an only child has many advantages, but as I get older, those self-serving advantages are less important to me. Family is what is most important now.