MN 137 - Directed only to Stream Entrants (and beyond)? "And what are the six kinds of renunciation joy? The joy that arises when — experiencing the inconstancy of those very forms, their change, fading, &amp; cessation — one sees with right discernment as it actually is that all forms, past or present, are inconstant, stressful, subject to change: That is called renunciation joy. (Similarly with sounds, smells, tastes, tactile sensations, &amp; ideas.)


Is this achieved by intellectual pondering about the six senses (and meditative insight into the three characteristics) or just pondering alone?

I'd say the former, but then this Sutta is primarily directed towards Stream Entrants, no?
The way my teacher explained a crucial point about this was summarized with a single but powerful word: Immediacy!

At some point in our practice our familiarity with Dharma should go beyond it being something "over there" that we study and try to understand, and become something very personal that is happening "right here" in our own immediate experience, second-by-second.

This type of joy that arises in case of mature student, is part of this breakthrough to the Immediacy of Dharma. On the initial phases of the breakthrough, we keep discovering more and more real life microsituations that turn out to be perfectly described by some ancient scripture, and this gives a sense of wonder and joy. And then as we get used to this match, we get a kind of relaxed bliss of a traveler who reached the oasis, had enough food and water to recover, and is now resting in the shade (to give a traditional image of what "bliss" means in this context).

Specifically as it pertains to Transience/Inconstancy of all configurations, once you clearly see that every frustration you used to take seriously is a temporary arrangement of clouds that actually shifts beyond pain fairly quickly, as long as you don't hold on to it, -- this immediate observation gives a sort of deep peace and a sense of relief that you experience continuously as you interact with all kinds of situations.

So to answer your question explicitly, it's not just intellectual pondering nor is it meditative insight, as it is the immediate observation (directly seeing) it all in your own life.