The development of the body from infancy is somewhat akin to the construction of a building.
It proceeds in an orderly way as appropriate building materials are moved about and fixed in place.

But from maturation onward the similarity ceases, and the body begins to produce more faulty materials - such as free radicals - than it can readily take out of the production line. This would be akin to bricklayers having to use inappropriate sized bricks halfway through the construction of a building. The building would be weakened and its longevity would be compromised.

The faulty materials the body must use after maturation, materials that it must incorporate into itself, cause faults with structures and functions in all tissues and organs. Now, instead of the orderly development that had proceeded from infancy, we have a disorderly development proceeding, and it is this disorderliness at the molecular level that takes us into aging.