As for extended life span of humans in Trek, maybe people just don't feel that it's necessary to push the average life span past it's natural point. If people are happy and able to accomplish everything they want in about 150ish years, maybe they just don't see a point in living past that.
This seems to be the ONLY credible explanation... however, we don't know whether 150 years is actually the limit to maximum Human lifespan.
As standards of living improve, so does life expectancy.
In Trek, it seems as if it just 'stopped' at about 150 - which realistically, doesn't make any sense.
And even if you argue that a lot of Humans may not see the need in living past 150 years of age, SOME Humans might want to.
I mean, exploring the Milky Way galaxy alone in DETAIL would take much longer than that.
I find it hard to imagine that explorers like Picard (who even complained at times about 'not having enough time') would pass up the option for it, considering we are talking about physical health, youthfulness and stamina to boot.
What we observed on-screen from various characters though, some would jump at the chance at living indefinitely.
Think about it... society changes constantly. This kind of change would be even faster in a futuristic setting.
By the time you lived a portion of your life exploring one facet of society, another facet would change so radically to the point where it would warrant you learn it all over again.
'Boredom' is hardly a viable excuse here.
There are plenty of things to do on a single planet, let alone this galaxy and universe at large... heck, even the multiverse.
The prospect for continuous learning and advancement is quite large, because tehcnical and scientific progress wouldn't just stop or go stagnant (the notion that short lives prompt progress isn't really a viable one - it may be a contributing factor, but driven people exist regardless).
As for using T'Pol in the 24th century... there might be a couple of reasons she would want to stick around.
Not every Vulcan is alike, and some aspects from Enterprise were shown that could happen in the 24th century that she might like to see through.