I would interpret "the latest possible date a prediction can come true and still remain in the lifetime of the person making it", "lifetime" would be the longest typical lifetime, rather than an actuarial average. So -- we know lots of people who live to 95, so that seems like it's within our possible lifetime. I certainly could live to 95, even if it's less than a 50/50 shot.

One other bit -- the average life expectancy is for the entire population, but the average life expectancy of white, college educated persons earning (or expected to earn) a first or second quintile income is quite a bit higher, and a very high proportion of LWers fall into that demographic. I took a quick actuarial survey a few months back that suggested my life expectancy given my family age/medical history, demographics, etc. was to reach 92 (I'm currently 43).