Is there a mod anywhere that changes the work hours for part-time jobs for young adults and older? That is, instead of working 4pm-7pm as a teenager would have to, it'd be 9am-12pm or similar for an adult working the same job.

I believe that would require creating new custom careers out of each one for the YAs and older, as the work hours for each career are universal across age groups. An alternative idea would be to use Buzzler's Moar Interactions mod to work a part time shift by command instead of according to set hours. But that, of course, wouldn't help if it was inactives in those careers you were concerned about.
http://modthesims.info/download.php?t=437344

Thank you, I already have Moar Interactions (and love it) but yes, am looking for something different.

I suspected custom careers might be the solution and had been reading through the guides here for them just in case. I might have a go at making them when my eyes stop glazing over from the abundance of information

Thank you for your help