Different people have different physiques, and there are congenital reasons and acquired exercise reasons, but the average level can be obtained through a lot of statistics and scientific analysis. And this average level can be used as a reference to compare the physical differences between a person and others of the same age, whether it is better than (young) peers or worse than (older) peers.
Just an example (inaccurate). For example, an adult male aged 30-35 can do dozens of push-ups a minute, and most of them can do 30, with 30 as the average standard (assuming that the average standard of 55-60 years old is 10). If you can't even do ten, it may mean that your upper limb strength is weak and may be evaluated as equivalent to the level of the elderly aged 55-60.
Of course, the above is just a hypothetical example. Generally speaking, the evaluation will not be so simple and idiotic. There will be many projects and evaluation methods. Comprehensive evaluation of a person's physique is very complicated and personalized. The final evaluation result is not absolute, and can only be used as a general reference for health, physique or sports level.