I read the whole article and the guy makes some good points. HOWEVER, IMO, carrying a knife for self defense might be ok for a younger man or woman. The average person over age 40 would be at a distinct disadvantage just due to the physiology of a person of that age. You just can't bend, twist, grab, maneuver, like a younger person and just the training may mess with your body. Oh, I am sure that there are people out there over 40 that are as nimble as a younger person but I believe they are in the minority.
I also carry a knife with me daily, and now the PA has finally made assisted opening knives legal, I carry one of those a lot more. But my knife carrying is for general use and is not geared to self defense. I carry a firearm for my and my wifes protection. Pure and simple I am way more comfortable with a firearm if things go bad than with a knife.