system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Sometimes a doctor says for example that a 50 year old person has the body of a 70 year old person because the body is in such a bad state. How do they determine that?
gpt
GPs see lots of 50-year-olds.  They also see lots of 70-year-olds.  If you see thousands and thousands of them over years of practice, you get a good feel for what sort of physical condition you can expect from the two age groups.  This can be based on things like general physical ability (how well can they sit and stand on their own, general fitness), their vital signs and body composition (in terms of weight and lean mass), lab values, and imaging (such as degree of osteoarthritis in a knee seen on X-ray).    They could also mean in terms of degree of disease progression, like quickly moving into end stage renal disease from type II diabetes.

It's not very scientific.  It's more a figure of speech than anything, and doesn't refer to any specific physical finding or laboratory values.