In the second example, oson is being used idiomatically to strengthen the superlative. When that happens, a form of dunamai has to be added. See Smyth 1086-1091, especially 1087. Moreover, this actually looks like an adverbial usage of oson, in which case a partitive is out of the question.
This seems to refer to the first example. There's no superlative; there's no form of the verb dunamai: δυνάμεως is the genitive of the noun dunamis, and depends on ὅσον. ὅσον is the subject of ἐστιν. "as much ability as I have".
Sorry, I got the examples backwards because of how they opened in my browser. Yes, of course there is no form of the verb. I was using a shorthand for what Smyth says: "a form, or a synonym, of dunamai". I don't know how I want to translate this example. I was incorrect to call it a superlative, but I take the matter of the idiom seriously because it pops up in other forms around osos and oios. I'll look for the references.
In the first example, oson is not a part of the lifetime/age/years. The genitive here is a rather bland genitive of quality. We are arriving at a point. Yes, it is a point in a lifetime, but that point is abstract and not a part of the whole. Socrates is talking about the youth's being at a certain point in his lifetime. He is not talking about some measureable part of the youth's lifetime/age/years. This I would argue frees up and indeed probably requires oson to be neuter since the point is indeed abstract (not having been explicitly stated other than through the pronoun oson).
Call it what you will, but ἡλικίας is a genitive depending on neuter ὅσον. He didn't write εἰς ὅσην ἡλικίαν, but he could have.
I don't agree. I don't think he could write that. That would mean he arrived at so much of his lifetime. But it makes no sense to arrive at a quantity.
This I would argue frees up and indeed probably requires oson to be neuter since the point is indeed abstract (not having been explicitly stated other than through the pronoun oson).
Not sure I see the distinction you're drawing here, but why isn't this true of ὅσον ... αὐτοῦ in the passage from Polybius?
No it is not true of the Polybius. oson in the Polybius passage refers to "as much (of the logos)". It is part
of the logos. Indeed, Polybius could have used an adjectival oson, oson logon. In the example from Plato, the youth has arrived at a point in his life. The moment perhaps belongs to the life. But it is not part of the life. You cannot rewrite the Plato sentence with an adjectival oson without gymnastics or destruction of sense.
If it were truly partitive genitive you could. Socrates isn't talking about a period of time. He's not talking about "as much time". He is talking about arriving at a moment. What kind of moment? A moment of time, in time. Time is a quality of the oson. It is a moment of time. It could have been a moment of forgetfulness. It could have been a moment of pleasure. Time is just a quality. But is not the whole from which a part is taken. Hence the genitive is not partitive.
I don't mean to be argumentative, either--I'm trying to be helpful--but I think you're needlessly torturing yourself to find an abstruse explanation for a construction that seems perfectly transparent.
I've been needlessly torturing myself since the day I took up Attic. Indeed, I thought that masochism was the point.
More seriously, I think it is far from transparent. You have yet to confront all the examples from LSJ where the gender of the part is the same as that of the whole. Is it your view that it is optional? That when one is using osos for a part, then one can either use the gender of the whole or the neuter? And what of oios? Do we have an option there also? And how about other pronouns?