The maximum growth rate occurs in:

In microbial growth curve, lag initial phase is characterized by cellular activity but not growth. exponential or log phase is the time when the cells are dividing by binary fission and doubling in numbers after each generation time. In stationary phase, the growth curve begins to decline as the available nutrients become depleted and waste products start to accumulate. The terminal irreversible stage of ageing is called senescence. This is the last phase of life span, that ultimately leads to death.
Related Theory Aging is a progressive decline with time whereas senescence occurs throughout the lifespan, including during embryogenesis.