As you go through your life on this earth, you come in to contact with so many people. Family, friends, coworkers, acquaintances, and even the people you watch on tv and movies become People You Know.
If you picture your life as a line, then all those other People You Know's lines intersect yours at some point. It could be they run alongside each other for your whole life, or theirs, or maybe they only touch briefly.
During that time when the lines touch, you KNOW something about that other person. It could be what they look like, how they act, what kind of music they are into, shared experiences, etc.
Maybe you watched one season of a show, so for that brief time you "knew" those actors, what they looked like at that time, those characters.
When your lines diverge from each other, what you know about each other may no longer be accurate. That old high school friend may have been a heavy-set person in their youth, but has since gotten more fit and looks very different. You may even have someone else tell you this, but you can't actually update the picture in your brain of that person without actually seeing it. In your head, they are forever remembered a certain way, and until you see them again your brain cannot concretely imagine them any other way.
This can apply to things as trivial as actors you love in a show, when the show ends and you don't see them again for a long time, then suddenly they are there and Oh My God they're OLD! And their hair is different?! They're fat now! When did this all happen?? It just seems like a minute ago they were young. Your brain just got an update on them, and has to process the new information to catalogue it and put it into the little box it keeps for that person. We were just watching the first couple episodes of Dexter last night, and Denise Crosby was on it. She played Tasha Yar in the first season of Star Trek: Next Generation many years ago. I haven't seen her since, so it was very jarring to see her older, with wrinkles and heavier set. Now that's how I'll see her in my mind's eye from this point on.
It can also apply more personally. Someone you know dies, and although you continue on, their line stops. My brother and mother are both dead. They are forever in my head as they were when they were alive. They will never age. They will never change. My brother is forever pulling coins from behind ears, and juggling, and going on enthusiastically about That game or That music. My mother is forever walking in the woods, pointing out nature to me, painting, forever suffering from various health issues. I clearly remember how they looked in death, but my brain rejects that as a temporary state, it wasn't what they REALLY were. They are frozen in time and will not get an "update" in this lifetime.
I don't think having People You Know frozen in time is necessarily a bad thing. It's just something our weird brains do as a way to keep track of stuff. It's a little sad that as you get older, this happens more and more as people drift apart or die. I guess that's just life.