The OP has no awareness of what they write. I (male) taught elementary for almost 25 years. I would never teach anything above that because the students were then bigger than me.
Teachers are trained at specific levels. It’s not that you know every grade level. We specify between early, middle or high (depending on how the state classifies it).
Students are NOT easier to teach as they get older. There goes that logic. No awareness. The younger students can be molded. They’re more of a blank slate to be worked with. It has its challenges but as for ease, I prefer elementary students. You get that “light bulb” moment more often.
Teaching little ones is “tricky and a serious responsibility”. Everything else shows a lack of experience and understanding of my profession coming from the OP.