That's just a statement, there is no logic behind why life would be experienced as percentages rather than in absolute time. Personally, I'm more prone to believe the unique experiences concept, because that actually has some basis for it, and also meshes with my real life. I have a few years in my early 20s which are just a blur (because I was working a crummy office job), but I have a year or two in my 30s which feel like a lifetime in themselves, and I can recall numerous specific days, because I was doing a totally new thing (road tripping around the mediterranean)