It's a rule for the range of people you can date.
If you are 99 years old...
99*(2/3)+7 = 73
73 - 99 years is the range of people you can date younger than you.

examples:

10 years = 10-13.6
14 years = 14-16.3
16 years = 16-17.6
18 years = 18-19
20 years = 20-20.3
30 years = 27-30
50 years = 40.3-50
99 years = 73-99

When you eat dirt for the first time and leave your family to build a shack in the woods