at what age (if ever) people start actively wanting to die soon
I don't think that wish to die is directly correlated with age -- I think it's mostly a function of the state of one's health (which is correlated with age, of course) and the consequent quality of life.
That's my intuition as well: that someone who lists 90 as their ideal life span while young and then makes it to 89 without obtaining some debilitating and painful illness will change their mind about dying at 90. The fact that older people list higher ideal life spans despite being less gung-ho on life extension in principle suggests to me that "wanting to die eventually" is Far. I wouldn't be surprised if there was no age at which the median person wanted to die soon, no matter what their youthful ideals about aging may have been.
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.