I think it has so much to do with your personalities, your child's personality, how you raise them, etc. I don't someone can say a statement like "a only child is a lonely child" and have it fit every child.
My DH is an only child and loves it. He says he never wanted a sibling. He sees how much I love (usually) having siblings and sees our bond and is happy for me - but he says he doesn't miss when he never had anyways. He said he liked growing up and getting all of his parent's attention. He said he learned to be more mature around adults quicker than most kids that he knew - as he was often only around adults. He was also close to his cousins who were closer to his age, who lived in town. His parents did lots of athletic activities with him and took him on lots of trips.
My SIL, on the other hand, is an only child (she grew up in Japan) and hated it. But she also had to do a ton of schoolwork and didn't have much time for play - so when she did have time to play, I am sure she would have loved to have a sibling around. So for her - having two kids (which they now have) was a priority for her.
We will most likely only have one child. It is mainly due to finances more than age. We aren't well off, and paying daycare is a struggle for us (and both of us working is the only way to do it in our case - believe me - I have run the numbers). So paying double daycare is just not an option for us. Plus we want to be able to retire, and also to be able to provide Charlotte with lots of attention as well as experiences. We love to travel and find a way to do it (we make it a priority) and it helps to have a very generous brother living in Hong Kong - who wants us to come visit and has and will give us frequent flyer miles. We have had many amazing travel experiences thanks to him - and we also have traveled extensively around the U.S. (we love car trips and hope Charlotte will love motoring in our MINI Cooper as much as we do - and so far, so good - she does great in the car).
And some say it's not fair to have one child - as who will take care of you when you are old? Well, I know plenty of families with more than one child - and none are "taking care" of their parent. Yes, they are helping to provide a place for them to live and help them move into that place and visit them, but just because you have more than one kid doesn't mean one will "take care" of you. And other family members could step up as well. Charlotte has a cousin close in age to her in town (2.5 years older) and two cousins in Hong Kong close in age to her - and despite distance - they are all close.
So whatever your reason is for having one child - whether it be age, fertility, finances, etc - don't let anyone make you feel guilty for doing what works for your family. Just do the best you can to provide your child with a happy life, and it will most likely be a great ride.
