Also, if the 20 year old looks back 5 years, they were a hormonal teenager, still growing physically, treated as a minor and had never experienced half the things they'd experienced now. They were a completely different person in many respects then, and if they look back 15 years it's their very first memories.
If the 50 year old looks back 5 years, their life probably wasn't all that different. If they look back 15, their kids were still at home and they had a different job title but they were fundamentally the same person, and they feel they've been that way for a very long time.
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)
1 year for a person who lived 50 years is relatively short(2%) compared to someone who lived for only 20 years (5%)