Why do you look older or younger in different kinds of lighting conditions?

I have had a number of people tell me that I look much younger than I am (53)-in fact not once but twice on my recent vacation sales clerks referred to me as a “young man”…

Now, when I look in a mirror, I can often believe them-but sometimes under certain lights (fluourescents?) I will indeed look my age. What, exactly, is going on to change the perception in question?