Every person you ask would tell you something different - the place where it started.
Some might mention the day they got their letter that brought them into a strange and fantastic new world full of strife, others the first time they heard two certain hyphenated names. There might be people who recall a certain battle or death. A precious few will think to recall the first time the elder of the hyphenated names first appeared. All will have a slightly different idea of when it started, and for them, that is when it did start, when it became real.
But, for one little boy (in heart, if not in body), it started when Tom Riddle feared one who was like him - a half blood - over one of the so called pure of blood. It started when he miraculously survived something no one had survived before. When he was placed on a stoop with only a letter for explanation. When a friendly giant became his first friend by knocking down a rickety door. And a gangly freckled-faced boy asked to sit in his cabin. And when an accidentally heroic act earned him a friend for life.
It all started one night when he realized he had something to fight for.
It started when this little boy, this young man, realized he had people to love and care for.
It started when he found something to live for.
It ended when he realized he had something to die for, to protect with his life.
So he calmly walked into the dark forest, not even noticing the pouring rain, he walked towards his death with no regrets.
When this old-before-his-time man's body was tenderly carried by his first friend onto the battle field, everything stopped.