We managed to reduce performance on any number of tests to essentially a single number, g, together with a couple more for domain-specific skill. We managed to reduce the huge variation in personalities to five numbers, the OCEAN dimensions. I even recall reading that there is quite some correlation between those five numbers and that they might be reduced to a single one but I can't find the source any more.

Can we construct a whole host of other, similar numbers, like "math skills" and thus measure the impact of education and aging?

Another number I have in mind is, can we construct three numbers general health gh, mental health mh and physical health ph, and measure their correlations? I have the vague observation that medical issues tend to cluster, that is people with mental issues tend to not only exhibit any one of ADHD, depression, OCD and so on, but more than one of them. Similarly I have the impression that people tend to complain of many physical symptoms at once.

I seem to recall that BMI and/or WHR tend to be excellent predictors of physical health. Together with a couple of more measures these predictions can further be improved. The advantage of having a single number would be for research purposes on population health and it is easier to have a single mumber for personal assesment.