Ideal Body-Mass Index In One Image (With Nuance)

From here.

The chart shows that:
(1) ideal BMI increases with age from about 18 at age 20 to about 26 at age 70.  For someone my age, the ideal BMI is about 23;
(2) low BMI increasingly becomes a problem starting about age 50 and becomes worse with time;
(3) significant marginal health risks from higher BMI start to kink at a BMI of about 37 at age twenty but drop to about 30-32 by age 70.

Putting together points (2) and (3) the BMI window around ideal BMI that doesn't have much mortality cost gets smaller and smaller from both ends, and the penalties for being the wrong weight are greater, as you get older.

As far as communicating that point goes, I'm not sure that one can do much better than the chart above.