Why duration of life of the present generation has shortened?

1. The protection of the brahmanas maintains the institution of varna and arama, the most scientific culture for attainment of spiritual life.  2. The protection of cows maintains the most miraculous form of food, i.e., milk for maintaining the finer tissues of the brain for understanding higher aims of life.
3. The protection of women maintains the chastity of society by which we can get a good generation for peace, tranquility and progress of life. 4. The protection of children gives the human form of life its best chance to prepare the way of liberty from material bondage. Such protection of children begins from the very day of begetting a child by the
purificatory process of Garbhadhana-samskara, the beginning of pure life.
5. The protection of the old men gives them a chance to prepare them selves for better life after death. This complete outlook is based on factors leading to successful humanity as against the civilization of polished cats and dogs. 
The killing of these innocent creatures is totally forbidden because even by insulting them one loses one's duration of life. In the age of Kali they are not properly protected, and therefore the duration of life of the present generation has shortened considerably SB 1.8.5