Young and old are antonyms, which means that the one is the opposite of the other (just like tall-short, fast-slow, etc)
Now, in these kinds of antonyms, we call one of them "marked" and the other "unmarked". In simple terms, this means that the one is neutral (the unmarked), while the other implies something (the marked)
In our examples, "old" is the unmarked and "young" the marked. Which means that when you say "John is as old as Jack", there is nothing special implied about it. But when you say "John is as young as Jack", the sentence implies that, for example, you expected John to be older than Jack, but he actually isn't. This is shown more clearly with sentences like these:
"How old are your parents?" (nothing implied; neutral sentence)
"How young are your parents?" (it implies that his/her parents look quite young)
In a sense, yeah. It is implied in a subconscious way.
Or, alternatively, it indicates that he is "young enough for something"
E.g. -Jack can go to the playground. Can John go?
-Yeah, he is as young as Jack
Meaning, he is young enough for the playground