Malnutrition can be suspected in anyone over age 65 with body mass index (BMI) less than 22. A desirable BMI is 22 to 27.A BMI over 27 suggests obesity.

According to page 303 of the book “Medical Speech-language Pathology: A Practitioner ́s Guide” (ISBN 978-1588903204) by Alex F. Johnson and Barbara Holcomb Jacobson, malnutrition can also be suspected in anyone over age 65 with body mass index (BMI) less than 22. A desirable BMI is 22 to 27. A BMI over 27 suggests obesity.

Info source: 
http://books.google.com.sg/books?id=wmSu-EI_PDgC&pg=PA304&lpg=PA304&dq=body+mass+index+less+than+22+is+malnutrition&source=bl&ots=rDbsZLL3Af&sig=1fk8R3T2nBvzoXfSLMa1afBkAQU&hl=en&sa=X&ei=fQ9TUYrpFoOIkwWbpYHoBA&ved=0CC0Q6AEwAA


http://books.google.com.sg/books?id=wmSu-EI_PDgC&printsec=frontcover#v=onepage&q&f=false