The 'Don't Know Why' singer believes reaching the milestone in March forced her to reassess her life and had a change in her creative approach to making 'The Fall', which has sold over half a million copies since being released last month.
She said: "It seems to be an age where everyone goes through changes. All my good friends have. Everyone's either breaking up or getting involved or married or having a baby.
"So when I made that album I think I was looking outside myself more at that time, because of where I was and what was going on in the world. The past couple of years, though, I've looked inward more."
Norah – who split from long-time boyfriend Lee Alexander earlier in the year – also says she feels no pressure to recreate the huge success she had in her 20s.
She added to the USA Today newspaper: "I don't feel like I have anything to prove now. My label actually was excited about me trying something new. I have a lot of support and a lot of freedom. It's a good place to be in."