I don't think that this quite matches the notion that learning is the brain's way of exercising, if learning/exercising as a child prevents aging more than learning/exercising as an adult.
Imagine if we could get all of our physical exercising in as children and become lazy as adults. ;-)
An alternative hypothesis might be that conversation is the brain's way of exercising.
It slows aging, not reverses it. If the first time you learn 4 languages or get into exercise is when you are 70, you don't immediately have the mine and body of a 57 year old
No?
Children who learn multiple languages don't necessarily become adults who continue to learn more languages beyond what they learned in childhood, so the analogy with exercise seems to break down.
Children learn the languages that are spoken around them. If it's one language, they learn one, if two or three, they learn two or three. This happens naturally, for all children, regardless of personal habits.