Interesting Facts About Lactose Intolerance
From Understanding Nutrition- 11th edition by Ellie Whitney/Sharon Rady Rolfes  (the text book I’m teaching out of this semester)

We all know about the signs of lactose intolerance, the gas, bloating, diarrhea and downright misery one will experience if they consume lactose.  They do not have (or have enough) lactase enzymes to break down these “sugars” and the bacteria then have a feeding frenzy at your expense.
Lactase enzymes are the strongest at birth.  (some are born without any)  As a teen we only have 5-10% as strong or as many enzymes at birth. 
Only 30% of the world’s population retains enough lactase enzymes to digest and absorb lactose efficiently throughout life.
The lowest problem is in Northern Europe/Scandinavian nations
The highest problem is in North America and African American and Asian descents.

Percentage of population groups who can not tolerate too much lactose or none at all.

>80% South East Asians
80% Native Americans
75% African Americans
70% Mediterranean
60% Eskimo
50% Hispanics
20% Caucasians
<10% Northern Europe

Sometimes it gets worse due to age, medications, intestinal villi damaged by disease.  This may be temporary or permanent.