1. The juvenile phase is the period of growth between the birth of an individual till reaches reproductive maturity. In plant, this is also called the vegetative phase.

2. The reproductive phase starts after the vegetative phase (juvenile phase) and the organisms produce offspring during this phase. A few plants show unusual flowering phenomena. Some of them such as bamboo species, flower only once in their lifetime, usually after 30-100 years. They produce large number of fruits and die. Among animals such as birds living in nature lay eggs seasonally while birds in captivity (as in poultry farm) can be made to lay eggs throughout the year.

3. Senescent phase or old age begins at the end of reproductive phase. It is last phase of life span during which there is progressive deterioration in the body and loss in the capability of reproduction. Old age ultimately leads to death of the organism. In plants, it is characterized by the yellowing of leaves and leaf fall.