One's Thirties
noun
You have the number "thirty", and age. So this refers to one's thirties.
さんじゅうだい
This uses on'yomi for all the kanji, so you should be good to go here!
あの男は三十代です。
The man over there is in his thirties.
三十代で、この会社に入りました。
I joined this company in my thirties.
三人の子どもが生まれたとき、父と母は三十代でした。
My parents were in their thirties when they had their three children.