I hear what you're saying, but gonna disagree I have less ability at 4 years old than I had at 2 years old....and certainly more ability at 18 than the day I was born. Have you ever watched a baby stand up or take it's first steps...they are inherently weak limbed in the legs, and their bone structure is not even fully developed until their early teens...at birth their knees and many bones in their bodies are composed primarily of cartilage (I would presume to make it easier to fit through that tiny outlet by which they appear). Interesting fact...they actually have far more bones than an adult human, they merge as they grow older into the standard 206 from the textbooks. Anyway....just think for consistency in your definition of abilities you have to ramp up during childhood/teen years in addition to ramp down as they get older.
Interesting that the average individual on your graph lives to 78, and you say the overweight person lives longer....but as far as I can tell the overweight person IS the average...but maybe I'm just being picky
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p.s. I am an engineer as well, and love your graphs