You have peak muscle tone around age 18/19, so even if you're the same weight now as then, it's probable that you were smaller then as muscle takes up much less space.
Body composition does change over a lifetime, particularly if you lose weight (fat & muscle mass) and then gain weight (just gain fat unless you've trained). So it might be that you have a higher % of fat than when younger and if you're remotely stressed or your hormones aren't optimal, then it's likely to be round the tummy.
I've never had a flat tummy and so I'll shut up now. :-)