Age dependent motor unit dedifferentiation is a key component of impaired muscle function in advanced age. Here, we tested the hypothesis that rat muscle histochemical profile during the lifespan of an individual has an age-specific pattern since comprehensive longitudinal studies of muscle differentiation after birth and dedifferentiation in advanced age are scarce. Our results show that extensor digitorum longus muscle (EDL) is comprised only of two fiber types after birth, type slow-oxidative (SO) and type SDH-intermediate (SDH-INT), the latter being indicative for the presence of polyneuronal innervation. In contrast to the constantly growing cross-sectional area of the muscle fibers, a dramatic decrease in SDH-INT proportion occurs between day 14 and 21 after birth resulting in a complete loss of fiber type SDH-INT at the age of 90 days (p<0.05). At the age of 270 days, the fiber type composition of rat EDL dedifferentiates as shown by the reappearance of the SDH-INT type with a further increase at the age of 540 days (p<0.05). These changes in histochemical fiber type spectra are brought about by fiber type conversion within the fast twich fibers. The findings of the present study provide further evidence that fiber type conversion is a basic mechanism leading to motor unit differentiation and dedifferentiation during ontogenesis. Fiber type conversion shows a distinct time specific pattern and is also characteristic for motor unit regeneration after peripheral nerve repair. Factors that influence fiber type conversion and thereby motor unit organization may provide a future therapeutic option to enhance the regenerative capacity of motor units.