Hi,
There was a request similar to yours about a week ago that you might want to check out (responses to) -'seeking input from moms with one child families' was the request heading (posted on 1/28 or 1/29, I think). Also, if you'd like, I can email you the web address/link to an Only Child parenting board (for even more input -from a bunch of great gals who I've been posting with for several years now).
If your reason for wanting a second child is solely coming from seeing so many families with more than one child rather than coming from the idea of having a another child be your own heart's desire, I'd defnitely take a lot of time and think it over. Whatever you end up deciding, in my opinion, if your decision is from your heart, it's win-win and you won't lose out on anything by having one child to raise or by having two to raise.. and your son won't lose out whether your family size is '3' or '4'.
If you decide to remain a family of 3, your age might be a great benefit in avoiding the comments from family, friends, even strangers that an only child family can sometimes receive (quite unsolicitously). Some of the younger moms on my only parenting board have had to deal with some flak and genuinely intrusive and insulting (and highly stereotypical) remarks regarding their family size and choice (often coming from extended family, let alone friends and strangers). The older moms have (happily) enjoyed a 'dying off' of such remarks as the years pass.
If it is your heart's desire and you decide to have a second child, I wouldn't worry so much about your age --unless your have pregnancy worries or health problems. Adoption is no 'second best', but an equally magical way to have a child (or a second child) and women in their forties can have healthy pregnancies and births --you've got options. --My mother was 42 when I was born (I'm the youngest of 8 girls).
One more comment, on having a sibling to play with and trust in adulthood: you never know what the sibling relationship will be. It could be beautiful, disastrous, or anything in between (in both their childhood and their adulthood). I'd definitely go into a second pregnancy or adoption process ready for the best or worst, ready for the child YOU want, not the sibling bond you hope for. Of course, if you want and have another child, I do hope everything does turn out to be all you hoped for. --And, I hope that if you remain a family of three, THAT turns out to be all you hoped for, too. Just look into your own heart; you know it best.
And, take care!