Years ago when I entered the field of philosophy I might have written exactly the same thing as you. Doing philosophy (well) means being willing to challenge those things previously taken for granted. This is exciting (eye opening) as well as, sometimes, troubling. If you are the only person you know who is doing this, it can also be lonely. I suggest that you find some philosophers to talk to.

Why is it important to rank which one is worse? And why expect one to be generally worse than another (rather than each case assessed on its merits)?

Your question about "best age to become a parent" seems to be asking about what is in the best interests of the child. A comprehensive ethical assessment of this question may and should include the interests of the parents (as persons, their interests are worthy of moral considerations). As for the best interests of the child, age of the parents is not instrinsically morally relevant. Age may be relevant in some situations if it is a proxy for such things as likelihood of living to raise the child or giving the child a good quality environment (older adults may have greater financial resources, or contrariwise less energy). There are so many things to consider in timing the birth of a child--if, indeed, one has such a choice, which is a recent luxury--that it is often difficult to tell what is in the best interests of a child. It seems ethically reasonable to postpone parenthood in situations such as personal financial crisis, personal illness, temporary physical and political dangers, lack of a co...

It is true that we can only see/interact/cognize as human beings do. But it does not follow that knowledge is generally "biased." Particular biases (that we can discover through cognitive psychology, social analysis etc and check for) may lead to specific faulty knowledge claims. We do our best: we check for the biases that we know humans make. Animals/extra terrestrials might see/interact/cognize differently. We can learn about them, and see how their knowledge might differ from ours. There is no perspectiveless point of view (some philosophers call it a "God's eye" point of view).

The salient feature of this question is that the dying person is an 11-year old child, rather than an adult. Our sympathy is aroused and we wish to protect the child from pain, especially the pain of knowing that s/he is about to die. I think it is important to first ask the question: if the dying person was an adult, would it be appropriate to lie about the prognosis? And I think the answer is "No. The adult has a right to be told the truth about their prognosis if they ask." So, if an adult has that right, why not a child? Some would say that a child "can't handle the information." But I think that depends on the child, and many 11-year olds might be able to handle the information as well as adults, and might use it to e.g. make final statements or requests. The work of anthropologist Myra Bluebond-Langner on dying children shows that children often handle the topic of their own death as maturely (or more maturely) as adults do. The beloved family doctor could probably have made a better decision.