Retirement dating in old age and a second or third career are all part of a complex network of policies and practices that are being called into question. which factor is the biggest influence in this shift

This question is incomplete. It is not clear about which shift it is asking. Meanwhile, judging on the source material of the question, it is asking about the shift in the occupations and lifestyle choices older people take.

Elaborating on the ideas in the ‘Development Through Life: A Psychosocial Approach’ By Barbara M. Newman, Philip R. Newman (1975), the perceived life expectancy makes people to shift certain stages in life and involved in new types of activities lifestyle choices at a later age, which was less uncommon for previous generation.

People also become more liberal and such activities as dating in old age and doing a third career shift is not considered to be inappropriate. With increasing number of divorces, older people take their chances with new relationships and just date as would only young people do in the past. Also, the desire to keep yourself cognitively and socially active prompts older people to take on new occupations.

they can increase to boost the enonomy.