I bet it's an effect from the correlation of age and religiousity.
I strongly doubt that given that the study found only an extremely weak correlation between religiosity and being against life extension, and by some religion related questions, no correlation at all.
Indeed, since the tradition maintains that Methuselah lived to be almost 1000, so there is nothing obviously wrong with a long life.
This new study by Pew Research on American opinions about radical life extension turned up some interesting results:
I also find the demographic splits on page 3 to be surprising. On the question of whether treatments to extend life by decades would be a good thing for society, whites are significantly less likely to agree: 36% of whites agree whereas 48% of Hispanics and 56% of blacks do. There is a negative correlation with age (48% of adults 18-29, 46% of adults 30-49, 37% of adults 50-64, 31% of adults 65 and older) and with income (47% of those earning 30k and less, 42% of those earning from 30k-75k, and 39% of those earning 75k+). The income result in particular surprises me, as my intuition was that people with a higher quality of life would be significantly more pro-life extension.