User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10901795-20130929163910/@comment-3146930-20130929183359

A lot of the times half-bloods don't realize they are half-bloods until they are around Percy's age in The Lightning Thief (11-13). Grover and Annabeth mention demigods that could live full lives because they didn't know they were demigods. Once you know you are a demigod, your powers start to develop and monsters start to take notice.

Yes strange things happened to Percy growing up (the snake and the one-eyed guy for example), but no one would be like "I must be the child of a god!" Percy's godly blood most likely didn't activate until he reached 12-ish and even then he could only move water a tiny bit in his anger. After that, the next time he uses his powers was against the Minotaur, and most of that was reflex. It wasn't until after he learned and accepted he was a demigod that he could freely move water and he didn't become wet when in water.