June 24th, 2020. Author's note:
"Q: June 24th is Do-Re-Mi Day. If you were to start playing an instrument now, whic would you choose?
A: I've been playing the ukulele for 20 years."

So, I double checked and Rumiko Takahashi mentioned in two separate author's comments during Inuyasha (one on April 14th, 1999 and one on December 26th, 2007) about wanting to learn how to play the ukulele (though in the 2007 one, she simply answered that if she were to take lessons, she'd take ukulele ones). So, apparently, she got started in 1999 after the author's comment or something.