She's in her 30s, it's perfectly normal for her to not have a big fanbase within the early teens. I wasn't jammin' out to Madonna when I was 10
As for the later teens, I personally know a 16 year old. She was born in 1994, she barely knows who Ayu is, haha. She knows that she's famous, but not for what she brought to the Jpop scene in the early 2000's.
Even for 21 year olds like myself, following Ayu from the beginning to now would kinda be the equivalent of still following The Spice Girls (if they were together) or Hanson since their debut.
Some people move on, some don't haha.
Ayu's music is very specific. It's definitely not trendy, if that makes sense. I mean, there aren't many popular singers making songs like Love song