1. Little RAM. As you upgrade the OS it requires more and more RAM until you reach the minimum (which is very slow) and need to upgrade.
2. Dust. Over time dust builds up and your computer runs hotter. When the GPU is very hot, it will lower its clock speed to prevent overheating (thus making it laggy).
I must disagree a bit. RAM has little do with speed in the first place. It can cause slow-downs as your drive caches when there isn't enough free RAM, but OS upgrades should not raise RAM requirements dramatically. For example, 4GB RAM should be good for 2-3 OS' to come, or longer.
Proof is that base MBAs ship with 4. If Apple is selling machines that should last the next 5-6 years (or at least 3), then they don't want to intentionally alienate those who have bought the machines with only 4GB RAM.
One aspect that can lead to a gradual decline is your HDD filling up however that is easy to monitor and control and should be normal housekeeping.
Just to clarify, it's a SSD drive. I point this about HDD and SSD's have different lifespans and different problems.
Hard Disks usually slow down due to gradual degradation, fragmentation... and that's all I can think of.
Solid State Drives slow down to filling up more than HDD's do; there are limited read/writes, so the less space there is, the less space for fault tolerance there is. Once a 'cell' dies, another cell can take the data's place, but if you have very little, it's hard to find room for misplaced data. Fragmentation is not a problem for SSD's because the read time is so quick anyways.
Both disks suffer from filling up because there is less space for caching.
But HDD's are slower in the first place.
Anyways, he has a SSD, not an HDD.