The majority of the process is controlled and purposefully done by the DNA. Your body grows in stages and goes through major changes from embryo, to fetus to infant, toddler teenager etc. Your body is developing mostly as intended. As we age though, our cells' DNA slowly breaks down over time and as cells continue to use DNA to perform lifer functions like making proteins or cell reproduction, the DNA is compromised over time and the cells cannot perform their functions as well and slowly perform worse until they start causing stress on cells around them and so on until a major bodily function fails.