Unless you can read ancient Greek, (which if so, I applaud you) you have to keep in mind that you're not reading "Socrates" (well, in any case you aren't, I assume you mean Plato, as Socrates never wrote anything), you're reading a painstaking translation of Socratic ideas into a modern parlance which, as vigilant and astute as the translators might be, is doubtless tinged with some word choices, re-structurings, and other elements that move the text more or less away from it's original form and meaning. In fact, these translation choices are often crucial. My copy of
De Anima has a nice translator's introduction that explains the difficult decision to translate the Ancient Greek 'psyche' as "mind" even though it meant something much closer to "soul" so that modern audiences would find the text more palatable. This is a crucial gloss on the translation and radically modifies (or should) one's reading of the text. Unfortunately very few people read translator's introductions.
So, I'm not so confident Socrate's conception of wisdom was so similar to ours. In fact, I'd feel much more comfortable with the hypothesis that it contained more dissimilarities, considering the material conditions Socrates lived under were radically different than our own—in fact we deal with a whole region of concepts, historical events, and technologies Socrates could have never considered. Yet his concept of wisdom is supposed to nearly sufficiently cover ours? What of the commoner's practical 'wisdom' about matters of economic success in a global market? Surely, you'd argue, that isn't capital W wisdom—ah but then we're in the realm of philosophical debate about what kind of wisdom we're talking about and what really constitutes wisdom in the proper sense (notice how this has absolutely nothing to do with the inert misapplication of statistical analysis or scientific experimentation).
I'm not disputing that wisdom points to something objective. I'm claiming that the objective thing it points to is subject to the same movement of history, and thus change, as anything else. That's not to say societies can't agree on what the concept means, but it's necessarily fuzzy, and potentially even fuzzy and ill defined within the confines of small groups. It is an interpretive concept with an indeterminate reference (unless you're a nominalist that wants to perform several painful acrobatic stunts to try and argue that it points to something like "the collection of all referents participants assign to the term wisdom in a given context C at a given time T." Somehow we don't often find such analyses very interesting or useful).
I'd also argue it's not even clear what wisdom means now. Just look at all the different takes on it that have cropped up in this thread. Point is, you can only begin to get at the meaning and the proper study of these concepts dialectically (something Socrates excelled at, by the way), that is, through discussion and holistic approaches that attempt to account for entire horizons of meaning and social interaction. Taking a statistical approach to this issue, sure, may give us an interesting little factoid we can whip out over dinner, but we hardly walk away from the findings feeling we've actually gained any knowledge about the concept. In this sense it is a constituted objectivity—not something that inheres concretely in the world. It's reference, so to speak, is necessarily indeterminate.
It hardly even has any utility. Assume it is true. Hardly anyone will much appreciate an argument for or against a particular decision based on the idea that the relationship between wisdom and age tapers off at 20. Furthermore, it's so divorced from any context that to even bring it up would probably miss the point of the issue and seem, as I said, idiotic.