I think another possible reason for the negative correlation with age is that as people get older and death becomes more imminent, they (in my experience) try harder to convince themselves and their peers that it is a good/natural/acceptable thing, to lessen the sense of fear and helplessness.

The median ideal life span is 90 years – about 11 years longer than the current average U.S. life expectancy, which is 78.7 years

I'd guess that there's nothing magically wonderful about living to be 90. The more important part seems to be wanting to live longer but not wanting (or wanting to want) anything that seems implausible or socially abnormal.

It would have been interesting to see if greater age was correlated with holding a stronger view either for or against radical life extension. I would predict that older people hold stronger views, since the issue is more relevant to their current life experiences.

Younger adults, to whom old age may seem far away, are more likely than those 65 and older to give an ideal age of 78 years or less (19% vs. 6%). The median ideal life span of adults under 30 – at 85 years – is lower than that for older adults.

I did not expect this. And it seems weird, since young people are also more optimistic about their futures. And more likely to want to undergo radical life extension. Plus they haven't suffered the effects of aging (having many loved ones die, illness and pain, etc.).