Well, ‘e’ only occurs before a direct object, so only in a predicate, where direct objects are, and thus not in a noun phrase, which cannot contain a predicate (since no li’, for example). I don’t think it is ever explicitly said, but it is implicit and implied throughout. Now, ‘pi’ does not only occur in noun phrases, but what ffollows a ‘pi’ is never actually a predicate, in spite of coming perilously close.
‘jan pi sike mute’ is fine and often used. 'jan pi sike suno mute’ is also fine (both, by different routes, from ‘jan li sike (e suno) mute’ by incorporation or not). Other “age” expressions are ‘majuna’ and ‘suli’ for “old”, ‘lili’ for ”young”. Oh and ‘pi sike (suno) (mute) lili’.