I have a hair that is only available for teen through adult and I'd like it for elders too but am not sure how you'd make a hair available for a different age group. I'm assuming I wouldn't need to edit the mesh in any way?
Hairs made for teen and adult work on elders as well, they all have the same head size/shape, so it's a shame the creator didn't enable it for elders. You can fix that yourself very easily using CAS Texture Unitool http://www.modthesims.info/d/364926 to recategorise it, but as this is a Windows program and you have a Mac, it's a bit more complicated. You can run CAS Texture Unitool on a Mac by installing a framework system called Mono which allows you to run programs that have been created using .NET (such as the Unitool) on Macs - information about it is here, under item number 5: http://www.modthesims.info/showthread.php?t=366580
Once you have Mono installed, you use the Terminal application on the Mac to run your app by typing mono [name of app]. If you have trouble getting this to work, you can post in the OS X Support Forum for assistance: http://www.modthesims.info/forumdisplay.php?f=552
Otherwise, you might want to use the PC you have access to, which will probably be a lot easier!
Once you do have CTU up and running, you open your hair file with it, then click the Part Category tab. In that screen, click Elder under the Age section. EDITED to add: Then click the 'Commit' button. Then just File/Save, and you're done.