Answer:
This is a very diffcult question. Logically, the best age to learn a language is as young as possible, like a baby or toddler. During these years, it is very easy for the brain to absorb information, and the information learned during this time will be easier to remember later in life.
(My grandparents are from Slovakia and didn't know much English when I was a kid. They would talk to me in a mix of English and Slovak, and even though I haven't formally studied Slovak and don't speak to them but maybe once every few months, I can understand the gist of our conversations over 15 years after they first met me.)
But there is also the problem of what the parents of these kids would want. Some parents might not want their kids learning a foreign language so young, or even learning one at all.
(I am learning Japanese using a language learning app. Some parents might not want children learning this because of our past with them. This could also be a problem with middle eastern languages, or even just simple Spanish, if parents aren't favorable towards Mexicans.)
So logically, you want to start language learning as young as possible, or the lowest age you can, but ethically, you should probably start older, maybe middle school or highschool, so that the children can offer more input about what language they want to learn so parents will be more inclined to let them study a language.
Step-by-step explanation: