Why is the erech (Vayikra 27:1–8) of a woman less than that of a man? And why does erech depend on age? Sources, please.
Rabbi Samson Raphael Hirsch's approach, if I recall correctly, is that the number 3 represents the family (father+mother+child), and an additional 2 represents the community. Men are obligated to be involved in both, whereas women are only obligated in the former.
(Note that the Torah's stage of "working towards completion" is ages 5 to 20, or 15 years. If we split that in half, we get two stages: 5--12.5, and 12.5--20. That fits well with the concept of bar/bat mitzvah -- half that time is maturing without the technical obligation, and half the time with.)
The Gemara does observe that retirement age proportionately devaluates a man more than a woman, hence the Talmudic saying:
Which sounds similar to the way one lady described having her retired husband home (I believe this is attributed to the Cosby show):
I don't have a source to cite, but historically speaking, since the ancient world revolved entirely around how to continue living, i.e. earning enough money to feed yourself and your family, and since women didn't need to provide income for the family like the men had to, therefore the monetary value of a working man was worth more than that of women, children, and the old. He had more working potential.