age thirty; one's thirties
Refers to the age of thirty or the decade of one's thirties. Often used in contexts like birthdays or life stages.
She just turned thirty.
Even though I'm in my thirties, I still feel young.
四十路 means age forty or one's forties, following the same pattern as 三十路.
Derived from 三十 (みそ, thirty) and 路 (じ, road or way), originally meaning 'the way of thirty' or 'thirty years of age'. The reading みそ is an older native Japanese counting form for thirty.