First of all, I do not believe that Pirkei Avos is halacha li’maseh, as it is not brought down limaseh by the rishonim/achronim. After all, we do not all get married by 18.
Second, there are many, many different of interpratations of what this phrase is refering to (as there often is in Pirkei Avos). For instance, I saw a pshat that this is a reminder that loshon hara is not permitted between spouses, something that is often neglected.