Keep in mind that the 80/20 approach was not invented for any purpose. It evolved over time as elite athletes strove to elevate their performance. Strictly speaking, 80/20 is universally practiced at the elite level today because there is no other way to get fitter. But part of the reason it enables these athletes to get fitter is that it is gentler on the body. Anecdotally, large numbers of athletes with an injury history report experiencing less pain and injury with the 80/20 method.
Regarding age, I spoke to Stephen Seiler about this and he said he believe 80/20 remains the best way to go throughout the athletic lifespan. It’s just that the overall workload may need to be reduced as an athlete gets older. But there’s no one-size-fits-all way to do this. One experienced 64-year-old athlete might be able to handle a Level 3 plan, while another might need to drop to Level 0. Listen to your body as you train. If you’re feeling good overall, there’s no need to adjust. If you’re not, dial it back a bit.