Zenith of human body is at like 28 yrs of age.
From there only downhill.
Yea, about so.
It's amazing how we have got such a relatively long life but only 20-30% of it is progressing. Rest is, as you say, only downhill.
We have created the whole society around our long lives. I mean, these days you have barely graduated and moving to work life when you are already decomposing. Makes no sense.
This is probably because we have extended human life artificially. It hasn't extended because of natural evolution but because we learned ways to keep ourselves alive longer with medicines, medical treatment, lighter labor etc. (some might call this evolution too, though). This has happened rapidly and our bodies haven't adapted by extending the progressing phase too. We have just extended the downhill phase.