According to Dr. Richard Frye, the treatment is also suitable for adolescents and adults. Here is a clip from an interview where he addresses this issue:
Interviewer:
I have a question from parents of children who are a bit older - ages 7, 10, or even adolescents.
I know that research is constantly changing and there are so many new things coming out.
Is it already too late for them to start such treatments, like leucovorin?
Dr. Frye:
Definitely not. It’s never too late.
I have treated children, adolescents, and also adults - and it’s never too late.
Link to the full interview. This segment appears at 21:15 in the full interview.