I always used to work on.
If you are flying in accordance with normal school guidlines ie
1.public cat aircraft
2. licensed airport (although that doesn't hold true now)
3. Under the oversight of a FTO
4. the person didn't have a license
You could log instructor with any person be they under 14 or over 100.
If the person did have a license you could log it in anything as long as I had access to the controls and I was legal to instruct on it in the particualr conditions. ie not if we went IMC then I would be PIC unless they held a IMC/IR and then they would log PIC or a tail dragger.
If it was a check ride for school reasons I used to let them log it as PIC. It seemed to remove a whole heap of animosity and pilots were more likely to go up for an hour with an instructor occassionally thus improving general safety. They didn't seem to begrudge paying for my time but the PIC bit seemed to raise folks hackles.
These days I try not to log it in anything unless the person needs a 1 hour or its a proper lesson with an instructor because it causes issues with FTL's .