That's the average. Some brain cells don't churn as fast (or maybe at all?). The memory-of-a-memory thing isn't on a 7 year scale either, but does happen. I don't remember the exact science, but I do remember the metaphor I came up with (evidence in my favor!).

Remembering something is like pulling a note out of a cabinet. It gets saved from the back, where the papers tend do degrade. Then, it gets put at the front. The trick is, the original note isn't put at the front. A copy is made. So remembering tends to reinforce, but also can change during this lossy copying process.