Am I going to be thin forever, or will I gain weight as I get older?

Some older people tell me how they were my size when they were my age, and 20 years later they’ve doubled in size essentially.

Obviously, my metabolism will slow down and I’ll have to start watching what I eat and exercise more, but is it really possible for me to gain a lot of weight if I’ve always been this 5’9 160lb guy? That’s a little scary.