Here is what I could fine on your question-

"Many researchers now use the following levels rather than the 4.0 used in the past. However, more time is needed to assure that these levels are more accurate.

• age 40 to 50, normal range is 0 to 2.5
• age 50 to 60, normal range is 0 to 3.5
• age 60 to 70, normal range is 0 to 4.5
• age 70 to 80, normal range is 0 to 6.5"