Actually I believe Harry was only nineteen or twenty when he left Eb. He was 16 when he was sent to live with him under the Doom and stayed for three years.. Not sure if he traveled around a bit or just when he studied for license and went to work for Angel Detective Agency and settled in Chicago. If I remember correctly in Storm Front Harry is twenty five or six..
Yep, I think it was confirmed in a WoJ that Harry was about 25 or 26 in SF.
He was 16 when he went to Eb, stayed there for 3 years, travelled around almost a year then went to Chicago. So he was about 20 when he became Nick's apprentice. The apprenticeship lasted 3 years, then he got his own office. Which he had for 2 or 3 years in SF.
Lets assume Restoration of Faith happened during the first year he was with Nick. Faith was 10. So she was about 15 in SF. That's about the age that magic can manifest. Assuming Cowl was the one who corrupted Victor Sells, he might have been around Chicago and met Faith. Cowl would only have taken her as an apprentice (rather than just corrupting her and see what flew up as he did with Sells) if she had real potential and was ready to take the left-hand path. Lets assume she was.
She was at Bianca's party with Cowl. Age 17. Not impossible.
In Dead Beat she would have been 20 or 21. That's also not impossible. The only one who saw part of her face was Lamar, the paramedic. He said she was white. Nothing about her looking a particular age.
Cowl and Kumori talked in English, even when they might not have wanted Harry to understand them. An older wizard might have picked up a language or two that was not so commonly understood all over the place.
Kumori's belief that Necromancy might be used for good is rather naive. Also fits a young person who got trained by a bad guy. And she was the one who tried to not kill Harry several times. Could be because he saved her once and introduced her to the world of magic in the first place.
The only complicated bit of magic we knew she did was the resurrection of Marcone's man in Dead Beat. Which was a bit stupid to do because she exhausted herself in a critical situation to save a criminal. A more experienced wizard would have had other priorities. This also indicates that she can't have understood the consequences of the Darkhallow - the guy she saved would probably have died anyway.
