Why, in the top age bracket, is the 'value' of a woman closer to that of a man than in the lower ones?
Rashi: This is in keeping with the saying 'An old man in the house is an obstacle in the house; an old woman in the house is a treasure and a good sign in the house'.
What is the highest age to which Erchin apply?
Rashi (Bechoros 12b DH u'Mosifin): There is no limit. 1
Moshav Zekenim citing R. Yehudah ha'Chasid: It is 72. We find that the Erchin until 20 years (five Shekalim from a month until five years, and 20 Shekalim for from age five to 20) are [together] 25, a fifth more [than the top age], and also from 20 to 60, it is a fifth more than the Erech (50 Shekalim), also above 60, the Erech (15) is a fifth more [than the number of years it spans, i.e. another 12 years 2 , until age 72). Moshe alluded to this (Tehilim 90:10) - "Yemei Shenoseinu Shiv'im Shanah", i.e. excluding two years of nursing. "Es Mispar Yamecha Amalei" (Shemos 23:26) - the Gematriya of Amalei is 72
The simple reading of the verse, Gemara and all the Meforshim (with only one exception - refer to 27:7:151:2) hold like this. (PF)
Each time, it is a fifth more in a different way! First, the sum Erchin for two age brackets is a fifth more than the top age of the higher bracket. (This is an outer fifth, i.e. a quarter of 20, the smaller amount.) Then, the top age of the next bracket is a fifth more than the Erech (unlike before, when the Erchin were greater! Also, now it is an inner fifth, i.e. a fifth of 50, the smaller amount!) And the last Erech is an [outer] fifth more than (not the top of the last age bracket, rather,) the number of years in the top bracket! How can we learn Torah law from a verse of Tehilim? Also, there is no source to exclude the years of nursing! Also, it is a fifth more only for males, and not for females! This requires great investigation. (PF)