In some questions, I include my age/grade. Answerers seem to treat me like a child (I use this metaphorically since I am). Is there a way I could request for them to treat me as if I were an adult?
Unfortunately, with some people, the second you let on your age, they will judge you for it and they may say some things that seem a bit patronizing or they may phrase things in a way that seems unfair. For examples like this, please know that we don't welcome that sort of attitude here, which can bee seen in a comment on the answer:
I would tend to avoid phrases like "there is no complexity to this situation at all" when people ask for help. I would also avoid overly vague solutions like "grow up", additionally calling religion, an important tenet in the OPs life, superstition indicated that you might not be objectively evaluating the situation and may be depreciating something that is important to the person we are trying to help. – Joe S
After this statement, the OP edited out their judgmental first sentence.
But I think HDE's correct in his comments here, pointing out that it wasn't your age that was attracting the reactive answers, but your question.
If you ever feel that you're being attacked for any reason - your age, religion, gender, political beliefs, sexuality - raise a flag. You can flag something as rude or abusive or, if you'd like a moderator to look into it themselves, you can raise a moderator flag.
Stack Exchange has the "Be Nice" policy, which rules the site. The first part is about this specifically:
Rudeness and belittling language are not okay. Your tone should match the way you'd talk in person with someone you respect and whom you want to respect you. If you don't have time to say something politely, just leave it for someone who does.
So, if it's outright rude, flag it or comment and ask them to change how they're phrasing things. You should feel welcome here to ask any question that fits our scope and be able to get useful answers that you can use.
Having read the question you refer to and the answers, I agree with the others who posted here that those comments reacted mainly to your somewhat contentious question and not your age, with the sole exception of the linked "grow up" comment which was later deleted.
I don't mind telling anybody here that I am 38, but it doesn't appear in my profile. We don't know the age of the vast majority of users on Stack Exchange network sites. Therefore we do interact with people without making any assumptions about their age (or gender, or nationality, or anything else they do not care to reveal).
So if you even remotely feel that anybody holds your age against you, simply remove that information from your profile and your posts! Since you are a native speaker of English your writing certainly doesn't reveal that you are a very young person, and nobody needs to know.
This almost feels like a question for the main site...
I know.it sucks to hear it, but age knows youth, but youth doesn't necessarily know age. I'm kinda old in internet terms... I remember things like dial up, and web safe colors.
When I answer questions that are posted by significantly younger users it can be hard to explain things that come with the experience of age without sounding painfully old. See my answer about sexiling a roommate... I'm old, but I can still clearly remember what having roommates was like, and how priorities were different at that age.
It may be somewhat unavoidable that age reflects in our questions and answers. A lot of the questions I see from younger users don't even mention the user's age, but their situation and perspective is very telling. As in, people usually don't get into or aren't usually bothered by certain situations as they get older.
On the other hand we don't want users to marginalize or minimize questions from younger users, purely because they're younger. Whenever you feel that an answer's tone is excessively condescending, or otherwise just not a good answer vote accordingly. We have the voting system for a reason, feel free to use it.
A little can be done when forming your question as well. It's not unreasonable to try to scope the sort of answers you're looking for. Ask yourself if your age is relevant to the question, if it isn't leave it out. When it is, include it, but make sure the over all tone of your question is mature. Sometimes sounding like an adult will lead people to talk to you more like an adult.