Tygre and I were talking the other day about some of our favorite fantasy universes and their metaphysical underpinnings (or lack thereof), and the subject came around to the fact that J.K. Rowling simply doesn't give a damn how or why magic works in the Potterverse.  And Tygre said something to the effect (I'm sure I'm putting words in his mouth now, so don't hold him to my interpretation of his remark) that this is particularly unsatisfying when it comes to the lack of any coherent mechanism to explain just how it is that Lily's self-sacrifice functions (and ceases to function) as magical protection for her son.

He's basically right about that.  I think that the Potterverse is fundamentally metaphysically incoherent and the ultimate explanation for the "rules" of that magical universe, such as they are, is narrative convenience.  But there is some degree of logic to the way that Lily's sacrifice and Lily's blood work beyond simply being symbols that self-giving love is a good thing.

For starters, it's not the protective power of Lily's love that wears out or expires when Harry comes of age, but only the bond between that magical protection and the home of Lily's sister, Petunia Dursley.  Why this is is never adequately explained (narrative convenience!).  But I think we can posit some sort of plausible theory that, as Harry is merely the Dursleys' ward and not their son, once he reaches his majority, their home is no longer legally, magically, his home, which is the predicate upon which the special protection of that place depends.  (Which makes me wonder whether, if the Dursleys had done as Dumbledore asked and accepted Harry as their own son, if they had loved him as Lily had, or even loved him for Lily's sake, the protective charm upon their home might not have broken when it did.  But they didn't, so nevermind.)

So the protection on the home expires, but the protection on Harry endures, and comes into play in the penultimate confrontation between Harry and Voldemort.

Here is how this works.  (Sort of. I think.)

Laying down your life for another is the opposite of a horcrux.  A horcrux destroys another life to extend one's own; self-sacrifice saves another life at the cost of its own.  This makes Lily's blood a kind of anti-horcrux.

When Voldemort regenerates his human body, he doesn't understand what it is about Harry that allowed him to survive the killing curse and defeat a powerful dark wizard, but he figures whatever that power is, he can appropriate it for himself by using Harry's blood in the regeneration spell.  When he later learns that it was Lily's sacrifice that protected Harry, he thinks that he has succeeded in co-opting that powerful protection for himself.  He doesn't realize that what he has done is infused his own very body with a magic that would rather die than hurt Harry.

So when Harry goes to meet Voldemort in the forbidden forest, he knows that he is a horcrux, carrying a fragment of Voldemort's "soul," and that all the horcruxes must be destroyed before Voldemort can die, and he is willing to lose his own life to make that happen.  Voldemort is ignorant both that Harry is a horcrux and that his (Voldemort's) body only exists by means of the anti-horcrux power of Harry's (i.e. Lily's) blood.  Therefore, when Voldemort uses his treacherous body to cast the spell that is meant to kill Harry, it only succeeds in killing the horcrux.  The power of Lily's blood in Voldemort's body would rather kill itself than hurt Harry, and so the killing spell cast by that body at Harry finds itself -- the bit of Voldemort's soul attached to Harry -- and kills that, rather than killing Harry.

Dumbledore insists that for this to even have a chance of working, Harry must be struck by Voldemort himself, and he must go willingly and not defend himself.  It is not clear to me why these are essential criteria. I think it has to be Voldemort because if anyone other than Voldemort-in-a-body-regenerated-with-Lily's-blood tries to kill Harry directly, Harry will simply die.  The protection of her sacrifice doesn't make him immortal.

The not-defending-himself part is harder to parse.  Is it simply so that the protection of self-giving sacrifice can be extended, casting that great protection over all those back at the castle for whom Harry was trying to lay down his life?  Or is that if Harry were to strike back, even in self-defense, that Voldemort-like behavior would have made it impossible, in that crucial moment, for the spell to distinguish the horcrux-in-Harry from Harry himself, and both would be destroyed?

Attempting to construct a fully formed underlying logic from the fragments of Potter canon that we have is an exercise in building castles in the clouds.  It is possible that I have now thought harder about these things than Rowling herself.  (I suspect not, though -- I think she probably did have at least the logic of blood vs. horcruxes worked out for herself, and just didn't put all her cards on the table when she wrote the books.)