How come Madonna, and the other pop girls who are in their 30's now can barely smash anymore but Rachel Platten managed to smash with "Fight Song" at 34 years old?

Is it because no one knows how old Rachel is?

No. The song is just really relateable and inspiring. It appeals to people of all ages and of all struggles and it's simply just a good song in that regard.

Because nobody knows her, so people liked the song and that made it a hit

Its because the song clicked to the GP taste. See the music video

Because it's a good song.

Madonna could have hits despite her age too, she just needs to release good music. Hung Up and 4 Minutes were huge hits and I believe she was in her 40's by that time.