Second Language Learning
Eric Lennberg created the critical period hypothesis which stated that the best time to learn is between 2 and puberty. There's this thing called laterialization which is when each side of the brain develops its own specialized functions.
It will usually take up to 2-3 years to converse and understand discussions and can also take up to 4 years to write, speak, read, and listen at an "academic level".
Older language learners have less of an advantage because of already having a 1st language to go off of. But they also have an easier time because they actually know whats going on instead of being totally confused.
Younger learners are more likely to have a native accent, and also have less to learn because they don't already have a language to go off of.