The reduction in percentage (%) of live BWG of broiler chicks due to the dietary lower protein level is reduced impact as the age of bird increased and corresponding figures are (6.25,5.67 and 3.49%) during starter, grower and finisher phases respectively. The explanation to that might be due that younger Cobb broiler chicks are characterized by fast growth (for building and configuring meat, muscle and skeleton tissues) and are more sensitive to the protein deficiency than the older ones.