Historically, ALPA has opposed raising the the age above 60 for decades.
Over the last 10-15 years a growing group of politicians, pilots, and even airlines has come to favor raising the age to 65. There are actually several valid reasons for this:
1) The original rule was created in the 1950's through back-door political deals involving the head of American Airlines and some of his political cronies. The sole purpose was to get rid of expensive senior pilots.
2) Since the 1950's the medical technology and health of the population at large has improved dramatically. In 1950 the average person was overweight, didn't exercise, smoked like a chimney, had bacon & eggs for breakfast, a ham sandwich for lunch, steak & potatoes for dinner, and washed it all down with a bottle of gin. Anybody can reasonably conclude that in the modern day we can expect at least an extra five years relative to 1950.
3) Other countries were going to age 65+
The rule almost got changed by congress to 63 in 2001 (pre 9/11), but the whole thing went on a back burner for a while after that.
ALPA did a major survey of it's members a couple years ago, and found that the majority were opposed to an increase (I voted in that one). ALPA then stated that it's official policy was to oppose age 65.
The really BIG change this year was the FAA. There are two ways to change the rule...the FAA can just change it (using some due process) since it is an administrative regulation. Congress can also change it by legislation which would over-ride FAA regulatory powers...all previous attempts to change the rule were all through the congressional route. The FAA never wanted to change the rule since they had nothing to gain, and possible exposure due to the fact the they couldn't prove that age 65 would not increase risk to the flying public...in typical beaurucratic fashion they opted to stick with the safe status quo. This year for some reason the FAA did a complete 180 and initiated the rule making process to increase the age to 65. I'm not exactly sure why, but it probably was a combination of things including possible pilot shortages and the ICAO rules (which allow age 65).
ALPA, under brand-new leadership, did a 180 also and decided to support age 65. Their excuse for this was that if they opposed the inevitable (I agree it is 99% inevitable) they would have no say in how the rule is implemented...so they chose to participate in order to gain some control over the details.
Since ALPA has a history of throwing the younger folks under the bus anytime it will benefit the senioir-most members, a lot of younger guys think this is just all about greed in the senior pilot ranks. The argument is that those guys benefitted from faster upgrades due to age 60, but now they want to get an extra five years at our expense.
Personally I think age 65 is reasonable, but I also think it should be phased in VERY slowly to prevent unfair career disruptions. By slowly I mean over 15+ years.