It has always seemed odd to me to hear people say "Wow. That was 10 years ago." or "Time flies so fast." While I empathize with the feeling, my inner response is usually something like "Every second of your life took place across one second, no?" So why does it feel like time moves so fast in retrospect?
The reason is that human memory uses a form of data compression. There is even some evidence to suggest that consciousness itself is an efficient compression method. All memories are partial, they are made up of fewer bits than their initial cognitive representations needed. If that were not the case, all memories of an experience would be virtually identical to having that experience again.
Here's a quick example
Say we have this data:
In run-length encoding, consecutive repeated characters are replaced with the character itself followed by the count of its repetitions.
Assuming we are using ASCII encoding, where each character is represented by 8 bits (1 byte):
Number of bits in Compressed data: 10 characters * 8 bits/character = 80 bits
To be clear, I am not merely making the obvious claim that "time passes at a constant rate". Instead, I am saying that due to the way our memory compresses information, the more time we are alive the more 'compressed memory' we will have and the 'faster' it will appear that time flies by. Our actual present experience is always full and our recollection of past experiences always partial. This creates the illusion that we should fear our actual present experiences 'slipping away faster and faster'.
It always helps to assuage the negative feelings I get when contemplating that "time moves quickly" to realize this simple truth. It helps me be present. That time moves any quicker than it actually takes place, as one accrues memories, is an illusion. Maybe that thought can help others too.