Perceptions

I've been thinking a bit lately about how I perceive others. I have this tendancy to assume certain things about other people. Like for instance age. I'm currently working two jobs where my colleagues vary in age from 19 to 60. when interacting with these people i, like most people do when getting to know another person, ask the general questions; how's it going?; do you have a day job?; and what's your age?
on several occasions i have been taken aback by the age of the person i'm talking to. in making my snap judgments (thank you Malcolm Gladwell) i just assume that this person is around the same age as myself. Yet when they answer 19, 20 or 21 i'm always shocked. Do these people look older for their age, or am i still perceiving myself as being 19,20, or 21 and erego don't notice the disparity in age.
now i know 21 is not all that removed from 25, but i figure it isn't all that hard to judge someone's age somewhat accurately.
This kind of perception had an opposite effect for those in an authoritative role. anyone, for instance the floor managers at the casino, who holds a higher rank to myself is automatically older than myself. So this generally comes as a shock to find out that they're younger or roughly the same age.
Anyways, i was just wondering if anyone else perceives people this way. I could write more on this subject but it is way too late in the night for further examples.