what is the healthiest age?

i think at least 25- 65  . 

20s on average is the physical health peak.

I think 18 to 25

I think 25 to 40 is the healthiest age. 

15, but you're under age and powerless, which combined with your health peak, spell inevitable disaster. But physical health is embarrassing without wisdom, to wit, l am still divesting myself of illusions at the age of 60. Sad but true...

Do you mean physical peak or least likely to die?

you can be healthy at ANY age. unhealthy, too. it has little to do with age. though over 50, the body tends to deteriorate more quickly.

you may not realize this, but from birth, the body is in a near constant state of dying. the skin cells on your body are completely replaced with all new ones about once a month.

eating a variety of foods, getting plenty of sleep, and exercising regularly, are still the best ways of staying healthy, regardless of age.

If you are talking physically healthy?  Somewhere around 14...if you are physically active and NOT overweight then. You lose some of your youth (and youth helps keep you healthy) every day that you are outside of the womb.  OTOH, there are certain illnesses that a child's immune system can't properly handle, yet.  By age 14, you are in that sweet spot, where "age" isn't starting to have a negative impact on your health yet, but your body is just mature enough to fight off most illnesses that would prey on younger children. 

According to an article in The Telegraph back in 2016, it was: 

Healthiest: 30. Wealthiest: 47. Happiest: 39 

35 I’m guessing