Obstacle is the opposite of Benefit. Sake is a type of Benefit. 
Girl is related to Youth. Youth is a different concept from Elderly. Elderly is related to Age. Age is related to License. 
Obstacle <--Antonym--> Benefit <--IsA-- Sake 
Girl <--RelatedTo--> Youth <--DistinctFrom--> Elderly <--RelatedTo--> Age <--RelatedTo--> License 