In your thirties
Definition 1 of 1
Pro Tip 1/3Life Milestones
The phrase can imply important life events or changes typically experienced during this age range. 
Many of my friends started families in their thirties.
Pro Tip 2/3Subtle Nuances
Terms like 'early', 'mid', or 'late' help specify more precisely within the decade. 
He switched careers in his early thirties.
Pro Tip 3/3Informal Planning
Used informally when talking about future plans without a specific age in mind. 
I plan to start my own business in my thirties.