Take this chart with the smallest grain of salt possible. This is bound to start a comment war somewhere on the internet. A recent article from Business Insider suggest that iPhone buyers tend to be younger than Samsung suggests.
For some time Samsung has been mocking Apple in its television spots, suggesting that iPhones are for your parents; the cool kids have Galaxy phones. It's a great ad, but apparently not entirely true. Apple has actually more customers from the age brackets of 18-24 and 25-34 than Samsung. Their devices also attract (not surprisingly) more people from higher income brackets, as well as buyers with higher levels of education.
These statistics follow another recent report suggesting that there are considerably more smartphone owners switching from Samsung phones to iPhones - three times as many to be precise.
It might be that Samsung's "unstoppable" growth has been fueled by first-time smartphone buyers. While that's great for short-run profits, it may spell trouble for Samsung if they plan on retaining their customers. Apple is by far the leader in customer satisfaction, so it's possible they will start to regain market share, at least in the US.