Escaped mysql function in join condition

Hi there!

I'm trying to use a BETWEEN condition when joining a table using
->leftJoin( 'ageclass', ' YEAR(user.birthdate) BETWEEN ageclass.startYear AND ageclass.endYear ', '', '')
but my YEAR-functions ends up in
ON `YEAR`(user.birthdate) BETWEEN ageclass.startYear AND ageclass.endYear

Any ideas how I can avoid the escaping of the YEAR-function or any better solution for my BETWEEN condition?

Thank you!