I have pondered this a lot, not only in a theoretical way, since my grandmother (who is 84 years old) is a devout but criminally uninformed Christian. She knows I am an atheist and usually respects my feelings--i.e., she only asks me about going to church occasionally! But I occasionally discuss religion and our varying beliefs, because I want to open her mind and make her more informed, both about her own faith and the myriad other faiths and philosophies out there. Plus it is hard not to get fired up given some of her views and reasons for believing (e.g., believing the Bible is true because it says she will go to heaven!).
Part of me would love for her to have an epiphany and give up her Christian belief system, but another part of me also feels a bit creepy about that given her age. Letting go of God is a hard thing for many people regardless of their age, and it could be much harder for someone who has lived her entire life with nothing else and little knowledge of any other options. Knowing how hard this could be, whatever benefits it might bring for her personally and/or on a larger level, I hesitate to say too much...
What do you all think about this? Is there an age limit non-theists should respect when it comes to engaging with theists?
(Disclaimer: My personal feeling is still "No," however awkward I might feel when I fight the good non-believer's fight...)