Harry's age when he first used magic/was adopted by Justin is referred to several times, and the first age given is different from most of the others.
Below are all the relevant quotes I can find. I think there is one where Harry says that Elaine was adopted a couple few weeks after he was, but I can't find it right now.
The gyst is that in Summer Knight Harry says that he and Elaine were adopted at 10. In Proven Guilty Harry says that Justin adopted him two weeks after his magic manifested. In Ghost Story he says his first time using magic broke the Iowa state high school long-jump record, and Ghost Story also says that he was 12 when he met Elaine.
There are also many mentionings of Harry's first lessons occuring around 13.
"We got adopted by the same man when we were ten.
"My fist time..." [/snip] It was maybe two weeks before Justin adopted me," I said. "I was in school and small. All elbows and ears. Hadn't hit my growth spurt yet, and it was spring
The memory of my first shielding lesson under Justin DuMorne wasn't a particularly sentimental one. "Baseballs."
Merciful God," Charity said, shaking her head. "How old were you?"
"Thirteen."
I broke the Iowa state high school long-jump record [/snip] I hadn't even gotten serious about puberty yet.
And I was nearly thirteen, so that was really saying something.
[/snip]
"Flickum bicus!"
I hadn't really been interested in girls yet when I met Elaine. We'd both been twelve, bright and stubborn"
We also have this WoJ on the matter:
How old were Eb and Maggie Sr when they first manifested a talent?
Same age as Harry–around 12 or 13.