Or Rapunzel, because I don't think she is very well-known among the older audience.

Many forget her name or just don't know her name.

HELL to the no, people know Aurora, they just dont know her full name. I did a survey on this, ( ill post the article soon) they know who she is. Just say Sleeping Beauty and she is known.

Mulan isnt widely known though. Or Rapunzel.

She is rarely included on merchandise. The original six princesses received heavy marketing when their movies came out and appeared on all sorts of merchandise. They are the ones most people grew up with and remember from their childhoods. The story of Rapunzel is widely known and before Disney put their spin on the tale, there was a Barbie Rapunzel movie. Frozen is the most recent princess movie, so Mulan is the logical choice.

What coolsinger said and queen said.

I'm still trying to figure out how Anna is still here. she's less known than even Mulan.

Or Jasmine.
@AudreyFreak Frozen is very well known so the Frozen sisters are well known too. Everyone knows "Do you want to build a snowman?" and "Let it Go". So they know Elsa and Anna really well.

She is not as popular in worldwide as she is in fanpop...