I'd say it depends; if you keep in decent shape and age gracefully then you're not going to be "old" probably well into your 60s, maybe 70.
I'm 35, and I look like I'm in my 20s. Part is I take care of myself, the other part is just good genes I guess. I don't "feel" 35, whatever that means. I know some people younger than me that look well older than me; I know some 60 year olds that play basketball at the Y 3 times a week with people over half their age. In a way, age doesn't really matter. Who cares if you're old or if you're young?