Hey fletch sorry didnt see your post till now
Right depending on what job your exping. If your leveling a meele or a mage job you can get level 12-4 solo.
Level 1-12
Any of the beginning areas so Ronfaure, Sarutabarut, Gustaberg depending on where your character was made.
12-14 If your on a meele job you can keep soloing its slow but you can use the experience point ring (empress band) bought from the gate guard in your country to speed this up. You can do this by soloing Decent challenges in La theine or konschtat highlands and meriphataud mountains
12+ On a mage job you need to go to Valkrum dunes you can get away going there at level 10 on a mage job but I wouldnt risk it i'd head there level 12 + on a mage job or lvl 13~14 on a meele job.
You can stay in the dunes till level 19~20 this again depends on your job mages tend to move from here around level 19 to go to Quifim Island. To get to Quifim you need to head to Jeuno. Once there you stay there till level 24~5.
Once you get level 25 you need to get the Kazham keys which allow you to use the airship to travel to Kazham where you will be looking at exping till level 30~2
If you need any help just give me a tell in game. Or want to know areas past that to exp let me know