Your body is a cannon, meant to shoot DNA into the future. Once it has fulfilled that purpose, and possibly done a bit to nurture the next generation(s) of your / your tribe's DNA (or harm the DNA of others), your body serves no more purpose.
There doesn't seem to be any obvious upper bound on this behavior.
I have a Harvard professor two centuries ago around 1800 in my ancestry. His DNA would still be serving a useful purpose to his descendants if he was teaching/advising my kids today in 2013, or if he was teaching/advising my descendents in early 2300 for that matter.
I would imagine being young in a post 500-year-lifespan society would be very suffocating if your first few decades had a student:teacher ratio flipped around from current numbers to 20 teachers per student rather than current 20 students per teacher.
The resources you consume, versus the reproductive edge your provide to your offspring. Specifically, if you're competing for resources they could have used in their own reproduction.
The most successful DNA will be when a group of people launch their descendants off into the galaxy to avoid the Red Giant death in our solar system. We're all rats on one slowly sinking ship, right now.
A lot of techno-utopia assumptions come into play, such as a culture powerful enough to extend life to 500 years probably has no resources left to compete for other than the momentary attraction of a fertile member of the opposite sex, although even that could probably be worked around in a lab (cloning or something?). If we discover something tomorrow I'd have to eat my words but I'm assuming it would require "star trek era" tech to pull this off.
