My 1rst class Zero I did completely solo including all sidequests in both main game and Scarlette DLC (only one that came out at the time on that PT been playing BL2 since release day). I switched to TVHM mode at around if memory serves me right Level 43. That included some grinding/farming for XP in areas I got was getting good XP.
I PL'd my next two classes.
Currently doing a co-op only run with my 3rd Siren (started 8 days ago) with a Live friend. We hit level 33 last nite. We are on hold with MS at Where Angels Fear. We have done all sidequests with exception of 1 in Scarlette DLC, Have switched to Torgue and done many sidequest but still have primary and few others to do.
So don't know how much you've done in to get to your level 42 but I can see on this PT of going a bit higher than 42 this time for me on this one. As for when to switch it really is up to you. From my past experience with Zer0 issue with switching to TVHM isn't your level as far as your character but will be a matter of gear on when you do. This is easily rectified by either going to DLC to get better stuff from vending machines, check other classes if you have any or just have somebody give you some better TVHM gear thats set at your level when you start. Doesnt even have to be legendary stuff but stuff set to your level.