The magic date

In the live of two persons there is only one day when the one person has the half age of the other person. Most people don't know this date and loose the chance to celebrate. Here you get this chance. I calculate it for you. Mark this date in your calendar.
cm
kg
BMI
< 16
16 - 17
17 - 18.5
18.5 - 25
25 - 30
30 - 35
35 - 40
≥ 40