I think it's partly an ingrained cognitive issue. There are some stable patterns to friendships and age, which this article is speculating around. Women tend t

I think it's partly an ingrained cognitive issue. There are some stable patterns to friendships and age, which this article is speculating around. Women tend to have more than men over 30, and it is true that the number of new one's fomed tends to decrease from the youthful stage, where other patterns are more prominent (practicing mature social roles through play, etc). Much of it is likely evolved, though there is definitely a large culural and environmental component.