In the US (and many other countries) the iPhone dominates every demographic except I guess the very poor? The whole "teenagers" thing has never been true. Grandma has an iPhone.

However worldwide Safari accounts for over 30%+ of mobile device impressions. That is an absolutely enormous market. It tends to be the market with the most cash in almost all markets (I'm not trying to be a booster or start some mobile platform war, and yes Samsung devices are super expensive as well, but demographically you can go to almost any country -- Japan, Russia, India, Brazil, just about everywhere, and the top n% are dominated by iPhones).

If advertisers aren't exploiting it it's either ignorance, or more likely that the dominant ad networks know it will get the ads flagged as abusive and Apple will drop the hammer.