There is no perfect metric that will tell you when you’re gonna die. There are metrics, however, that are associated with certain types of disease, and many associated with so-called “all-cause mortality” - which includes everything from disease to cancer to falling and breaking your neck.
All of these metrics naturally tend to decline with age. You can work to slow this decline, but eventually death comes for us all.
Grip Strength
Grip strength is perhaps more predictive of all-cause mortality than any other single metric. It’s probably mostly as a Proxy Measures for overall strength, but grip strength itself is also vital for things like preventing falls and handling day-to-day life activities.
VO2 Max
V02 Max is next in line behind grip strength.
Insulin Resistance
If your body isn’t capable of utilizing insulin, that’s bad. It’s a pre-symptom of Diabetes.