How valid do you feel this statement is? Every five years you can look back and see how much you have changed as a person.

I think it's mostly true.
The rate of change usually slows over time though. I think people change more between 20-25, than 60-65.
Our personalities and beliefs are usually malleable in our youth, but become more refined, and ridged as we age.
So, maybe a logarithmic spacing instead of set intervals of 5 years.

On the other hand, it does not apply to everyone. A person needs to have some ability to self reflect with an attempt at objectivity.
That is difficult for some. And impossible for others.

I'm mostly familiar with that idea through the Red vs Blue PSA on tattoos.