Probably one of the biggest earliest ironies of The Last Olympian. Is by Chiron not ever clarifying the 'Shall Reach 16' thing to Percy. Is that inavertidely got Percy doing the one thing Percy probably wouldn't have done under normal circumstances. Accidentally. Because he didn't have all the correct information.
Which was trying to change the prophecy.
Cause first he tries to engage Kronos first thinking he can beat Luke because he does not understand that it's not a matter of he needs to beat Kronos around 16. No he has to be 16.
And then there's the thing in Atlantis where he thinks he can make the decision to stay there to help his Dad also at the time. Which if he had stayed down there long enough would have eventually qualified actually.
But still didn't fully work at the time again because he wasn't actually 16.
But again he was accidentally engaging in a Greek Mythos No-No. When he usually doesn't. Because Chiron despite knowing about the Andromeda plan didn't think to maybe give a carefully worded warning to avoid Kronos and it not being time yet.