This is one of those areas where the terminology and the processes have been so bastardized over time that it can be quite confusing to differentiate between the different forms and variations of preferments. I personally tend to use the “preferment” terminology as given in the Pizza Glossary at http://www.pizzamaking.com/pizza_glossary.html#P
. In a fairly broad and general way, the Glossary preferment definition would cover things like poolish, biga, sponge, pate fermente, and old dough, whether they are based on commercial yeast or not, although they are most commonly based on commercial yeast. To be technically correct, I would except from the definition the use of starters that are used mainly for leavening purposes, as is commonly done, for example, with certain Neapolitan doughs that can use very small percents of starter (e.g., 1-5% of the weight of the formula water), and no commercial yeast.
While the hydration of preferments can vary from one form to another as you observed, where old dough tends to be different from the other forms of preferments is in the fact that it includes salt. From my research, I have identified two forms of old dough. In one form, which I personally call “new” old dough, a piece of dough made from flour, water, yeast (commercial yeast) and salt is allowed to ferment for a specified period of time before it becomes part of the final mix. As with pretty much all preferments, one wants to get the maximum flavor contribution (and other favorable attributes) from the old dough so it is common to allow the old dough to preferment for several hours, with a typical period being 3-6 hours at room temperature. If a longer preferment period is desired, then the usual way to achieve it is to let the old dough preferment for an hour or two at room temperature and then refrigerate it for a period of up to 48 hours (at around 35-40 degrees F) before making it part of the final mix. In this latter instance, you will want to allow the old dough to warm up before incorporating it into the final mix, or otherwise use warmer water in making the final dough as a way of compensating for the cold old dough.
In the second form of old dough, part of one day’s dough is saved for the next day’s production. In the bread baking world, bakers tend to use baguette dough for this purpose because it is composed of only flour, water, yeast and salt and, hence, can be used for just about any bread. Typically, the old dough that is saved comes from the dough batch just after the first fermentation. It is then usually refrigerated. Some pizza operators use dough that remains unused at the end of the day as part of the next day's dough batch. I refer to that dough as "scrap" rather than old dough because it may have limited leavening power and be approaching the end of its useful life. For those operators, using scrap dough is mainly a cost saving measure, although there may be a side benefit from the flavor contribution of the scrap dough. Even then, its use is limited to less than 15-25%, depending on where it is in its normal cycle.
Formulas for old dough can vary quite significantly, although in the bread baking world 40-50% is a common proportion (based on the flour of the final mix). But it can be as low as 10% or over 100%. In the “new” old dough method mentioned above, a common amount is 20-30% of the total formula flour, with a typical hydration of 64-66%, salt at 2%, and yeast at 0.3-0.5% (IDY). The hydration, salt and yeast are with respect to the weight of the flour used for the preferment. For pizza dough applications, I think I would use less salt.
I have done some limited experimentation with the concepts of old dough, both the “new” old dough (using natural starters) and old dough saved from an existing dough, and achieved pretty good results. However, working with old dough requires substantial attention to the process and use on a repetitive basis, as professional bakers do it as part of their business, especially those who use the saved old dough. For me as a casual user, it is a lot of work.
You might also be interested in reading Marco's (pizzanapoletana) comments on the subject, at http://www.pizzamaking.com/forum/index.php/topic,861.msg8679.html#msg8679