Just spam fates, you get an extra 50% exp for having a higher level class. That being said you can go to a different starter zone and do quests there too.

15-20 (aleport)
20-30 (quarrymill/southshroud)
30-34/35 (costa del Sol)
35-43/45 (Cortheas Highlands)
43-50 (N. Thanalan)