I am confused about why this is considered an issue. I am new to this StackExchange, so it's possible there's context specific to this SE I'm missing but I agree with Robert Soupe's position, though I think his answer could have been more "eloquently" presented.
I mentioned in a comment to B. Goddard's answer that more context is in general better. Not just for people attempting to answer the question but also for search engines and Stack Exchange to find relevant results. If someone says they're in 8th grade, that means any answer should take into account what they would or wouldn't know on the subject. (Though I think requiring more information than "8th grade" is a good idea, because 8th grade can mean different things in different countries and not all 8th graders are taking the same level of math).
There are two primary possibilities when someone claims they are in 8th grade and in both of them it really shouldn't matter:
They are not \$age
If they are not, I don't really see how it makes a difference. A bad question is still a bad question and a good question and a good question is still a good question. We already have a way to deal with bad questions: downvotes and edits. And if a bad question ends up with lots of reputation, the only people really at fault are the people who upvoted it. And I find it unlikely people would upvote bad questions just because they include 4 year olds.
And questions showing up on other SE sites shouldn't really have much of an effect. Voting up questions and answers requires 15 rep, reaching this level would mean the user either received 15 rep on the Math SE site or 100 rep on any other SE site. Either way, a user would either be familiar with acceptable conduct on this SE site or somewhat familiar with what is and isn't acceptable on Stack Exchange network sites as a whole. It also of course ensures they likely aren't a bot, but I don't think there's much of a motivation for that anyway that wouldn't already include breaking other rules.
If you have hard proof that someone is lying (i.e. a post to the parenting SE proving where they contradict the existence of their 2 year old) then absolutely call them out on it in the comments or on meta and downvote them. But unless you actually have proof of someone lying, I feel you should typically assume good faith, even if their claims are highly unlikely. While a 4 year old might not understand advanced math, it's possible something they said made their parent realize something, and they just neglected to include that.
They are \$age
I also don't think this situation should be much of a concern. To use the example mentioned by @B.Goddard, if an 8th grader has a question regarding a why their solution to an impossible problem is incorrect, they likely are pretty interested and eager to study math, and it's probably best not to actively discourage them. In the case of angle trisection that B.Goddard used as an example, it's possible they misunderstood part of the problem due to a bad or oversimplified explanation. If their question is original, it's valuable even if trivial and if their question is a duplicate, it should be flagged as such.
The Stack Exchange network already has a somewhat negative reputation for how it treats novices. While I don't expect every experienced user is obligated to take their time to answer novice questions, I also don't think it's acceptable for experienced users to actively discourage new users. A novice user today might someday end up as a valuable contributor to this site, another site on the network, or mathematics as a whole.