For Human subjects if there is any age ranging ?how do you say High fat diet ? high fat ok ,how can we measure they are in diet or not ?

Humans do not specificly place themselves on high fat diet neither would an investigator be allowed to place other humans on a high fat diet irrespecive of the  age of subject or time frame of the study. However there may be ethnicities whose diets can be considered high fat, them you may need to find.