I realised something

Well, to be specific, he fucked himself up, splitting his soul in 8 pieces and all that... 

In neither case was he actually defeated by Harry. The first time it was Harry's mother. The second time it was the Machiavellian machinations of Dumbledore, who carefully arranged his own death in such a way that Harry would have won even if he didn't flippin' know what was going on and just randomly bumbled into a fight with him. (Altho Harry *did* extensively contribute to ensuring he stayed dead afterwards).

If you're into HP fanfics, btw, allow me to recommend two: HPMOR and For Love of Magic. 

First, he was defeated by the Power of Love associated with Lilly's Sacrifice on the coincidence that Snape wanted him to spare her, which empowered Love's Protection to where the rebound of the Killing Curse got him. 

Silly thing is he waited till Harry is in school. If he done it in middle of breaks Harry has no chance once he return if there is a school still standing. Then, he could easily defeat Harry as he no longer has any support. 

It doesn't matter whether it faded at 17 or not. I suspect it did, actually, but it was utterly useless after the Goblet of Fire either way. Voldemort took on Harry's blood precisely so that it would no longer protect Harry from him, and it did not. That, then, was that.

IIRC, he also took on that blood because it was a requirement of his resurrection, being "the blood of an enemy".

By the end of the books, when they finally did fight again, the protection most likely had worn off anyway. But it would not have played any role in their battle regardless.  --- Or, rather, they technically fought a couple of times before that, but with their wands interfering. Which, funnily enough, is sort of what happened in the very end, too. Voldemort had no luck with wands! 

 

Once again the power of plot is at work  

The protection still existed, since its inherent to those consisting of those who carry the protected's blood. Harry's Blood runs through Voldemorts veins, Voldemort could never defeat him. Why do you think Harry grew up with his Aunt. Because of the Protection it provided due to lasting with blood relatives, it was supposed to end at age 17. But considering Voldemort had Harry's blood flowing through him, it didn't fade.