I was a student for half of my twenties. I should start working
I was student for the first half of my twenties. I should probably start working.
I spent the first half of my twenties as a student...
I was a student for half of my twenties = 20代の半分は学生だった。
I was a student for the first half of my twenties = 20代の前半は学生だった。
I spent the first half of my twenties as a student = 20代の前半は学生として過ごした。
尚、「__代」と言いたい時は “my, your, his, her, our, their” を使うべしです。
“My twenties” (私の20代)
“I should..” = した方がいい。するべき
“I probably should” =した方がいいかも。するべきかも。