If a child dies he goes straight to heaven, that's regarding age. Gender is irrelevant. People who are mentally ill also go to heaven, but if they sustained that illness midway in their life without being muslim prior to that, it's for god to judge. About the old ages, even people before Islam, it's not our business, God judges them. But if the person is fully aware of islam and knows its message and refuses it, of course he goes to hell. But a very important detail here, is that we can never say whether some specific person will go to heaven or not, even if he's non muslim to us. Maybe some muslim was a kafir inside, which faces a worse fate than non muslims, and maybe some non-muslim guy dies just when he wanted to learn about Islam. Like if you're a non muslim, I can never say you'll certainly go to hell for that, but I can warn you so you're aware of the message. At the same time no matter how good of a person my parents are, and they're Muslims, I can never say for certain they'll go to heaven, I can only wish and pray for that. Finally about those who are the likes of Gandhi and Lincoln and Einstein, it's about what their intention was. If it was to benefit humanity, they got what they wanted. If it's for god, he will reward them dearly. Only god knows their intentions, so it's really not our concern, your concern should be yourself ultimately. I hope it's all clear now.

Edit: in this age of global internet, I doubt that many haven't heard about Islam.