(a) elementary education in my state is K-6, so we are talking about 5YO to 12YO ... that is a huge range. A 12YO can do a lot more than a 5YO.
(b) whenever asking about what to teach, step 1 would be to consult state standards. If your state has none, consult NGSS. This is a middle school standard in NGSS (ESS3), listed at grade 6-8. In MN, it appears to start in HS - 9E.220.127.116.11
(c) I can't help wondering if this is "I'm a climate denier, and I'm looking for an excuse to not teach something against my political beliefs" - forgive me if I am wrong, but I have seen enough like this phrased slightly differently at the high school level for climate, Big Bang, evolution, etc. to not have to ask the question.
I think the topic of climate could be used to talk about supporting an argument with evidence (using a C.E.R. framework) in upper elementary - not the mechanism. What is science? One of the hallmarks of science is that it falsifiable. Moreover, what evidence would it take to disprove a hypothesis/argument, and how to write a good hypothesis. I love the GRIST (https://grist.org/article/skeptics-2/) site that goes through all the climate denier arguments (from sunspots to heat islands) and dispels them using data. For example, in the early 2000 the increasing solar activity did correlate with climate data ... but then sunspot activity decreased and the climate data continued to increase. Or the argument that all thermometers were in cities that are growing, cities are heat islands, as the cities grow, they have a bigger heat island effect and this is all we are seeing. But as those of us who live in rural areas know, many of the smaller towns in the USA are getting smaller as people move to the cities. And these shrinking towns are seeing the same temp rise as the cities. There are many good examples of this, and this is a skill that is part of science literacy.
The other concern here, though is that if you know anything about moral development stages, kids go through stages and they are black and white when young. While older people may still fall for fallicies, if the kids hear negative things about climate change at home it is going to be even harder with them to deal with something that might conflict with what they hear at home.
Again, sorry if I was off-base on C.