Well... it depends how hard you're wanting to try.
If you don't already have them, you should get into some healthy habits, like doing some type of physical exercises, or eating the right foods. If you're not really that overweight, and it's just your belly, then you may just want to work and tone your belly.
Indeed, it does depend on how tall you are to decide how much you should weigh. Age has nothing to do with it as long as you're an adult (or close). Look at this chart, find your height, and this will tell you about how much you should weigh.
http://www.momswhothink.com/diet-and...ght-chart.html
If you're actually fairly overweight, then you shouldn't worry about just your stomach, you should worry about everything else too. Just worrying about your stomach isn't going to get you anywhere long-term.
So, like I said, maybe start being more physically active. There's all sorts of ways to though, it doesn't have to be a chore. You could go for walks, you could ride a bike, you could go for a swim, or you could just dance around! Get your blood flowing a bit more.
Hope this helps you out, and good luck!