Easy. Enjoy life, enjoy all those little pleasures that are either immoral or you get fat/sick from them. Drop dead before the onset of senility. QED.
Every single male from my extended family during the last 2 generations has dropped dead from heart attack/stroke way before they turned into barely moving lump of protein requiring 3 nurses, 2 iPads and a mobile toilet to "live". Me, with my pack a cigarets a day - I am expecting the same fate. Long life is not as nice as people think...so no, I have covered that angle (ergo, no need for children to take care of me).
If am wrong, I've covered that angle too - as a scientist and a person with enormous interest in all kinds of subjects I keep my mind very busy, so no senility for me. Thus, when the body really starts giving up but the mind is still clear...well, meet my little friend - 2L bottle with compressed nitrogen and a face mask. You cannot fire me, I quit! (bonus: no (grand) children will be hurt by my action).
Rationality - can't beat it! So join it!