It is a little bit correct when it says that regret is not a useful reaction to a past you're unhappy with, but even that by itself is misleading. Regret is a useful emotion that helps you shape future actions. What is not useful is paralyzing regret, or any flavor of regret that keeps you wallowing in the past.
When he says "you are not behind", that is mostly wrong. If you're 25 and aren't yet doing anything individual and attemptedly groundbreaking with your life, you probably are behind, if that kind of thing is your goal. Sticking your head in the sand is not going to make this better. Being complacent and saying it's fine, I am only 25, no wait 26, no wait 27 until you are 40 isn't going to help either.
There is a reason the human mind is able to conjure phantasmal pictures of "where we should be" -- because that is useful. If you choose to ignore that in order to have a shallow feel-good time in the short term, you do so to your own detriment.
All that said, if you are genuinely content with where you are today, then everything is fine and you don't need externally-imposed images to tell you where you "should" be. This advice is only for people who deep-down want to build interesting new things.
My last comment is ... this seems like an excerpt from a self-help book written by someone who perhaps should gain further life experience before writing a self-help book. When you decide to write a self-help book you take upon yourself a substantial ethical burden, because if you give the wrong advice, you can affect many peoples' lives in a negative way. So you should make sure you really know what you are talking about.