system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that you never notice a person (mostly kids) aging when you see them regularly, but when you look back at old photos/videos, you only then realize how much they've grown?

I'll give my cousin as an example. He's in his "puberty" years, so a lot of bodily changes happen then. To me, he's always looked the same  as he did a few years ago. I only really noticed a small change in his voice, but nothing major. Then I look back on old home videos, and saw how truly young he looked before "puberty hit him". I never noticed the change until then. Could someone explain why this happens?
gpt
Day to day things are stored in short term memory. The mundane gets replace a lot. Long term memory is stored elsewhere in the brain. Looking at an old photograph will bring those memories to the short term part of the brain where then the shock process occurs. 

Other forms of this happen when you get older, move a lot, or just are away for a long while. 
Example, say you live on the east coast of the US and you move to the west coast. You lived on the east for 20 years. While you go about your daily life on the west coast you will see people that look just like people you know from the east coast. If you could put them side by side you would see they usually don't look alike. It is just your brain trying to comprehend the radical changes that took place with the move, and loss of familiarity. 
When old people get senile the brain will do the same thing. Trying to place old memories to what it can't comprehend in the short term. Grandma might start calling you by a late relative's name for example.