One of the things that fascinates most about history is that there were so much fewer people on the planet not too long ago. I'm generally good at empathizing, but I find it very hard to wrap my head around this aspect of say the 1700s.
This morning I asked my Dad whether he has noticed any changes on this axis in his lifetime. I haven't (born in 79'). He has though. He said that up until the mid 80s when he went out (first in NYC, and then in Atlanta), even to the airport (in this case, Atlanta), he would more often than not see multiple people he knew and think nothing of it. Fast forward to today and going to the airport and even the supermarket generally results in not seeing anyone he knows, even though he still lives in the same place.
These demographic changes relate to picking markets in startups. Consider the growth in India in Internet and mobile payments that is currently happening. Thousands of online businesses will spring into being, riding this wave. Yes, Facebook et al. will be big, but opportunity abounds for smaller players too who are catering to this market.
Or consider the well-known-by-now example of the app stores. I saw on Hacker News this morning the announcement of Leafsnap, an app that lets you take a picture of a leaf and tells you what tree it comes from. I told my Dad and he was immediately interested in using it, already having trees he had been waiting to identify. Then this comment caught my eye from a potential customer saying they'd love a whole line of these, for bugs, animal footprints, etc. that they could use as a suite on hiking trips.
Would this make a $100M dollar company? Probably not, and as an investor I'm still very wary of app companies for various reasons. But would this make a great business for a few developers? Absolutely. And I could see REI or EMS or whoever buying up the company for $20M.
I haven't checked, but I would bet that in the 60s a higher % of people read comic books, but today more people in total read them. There are just so many more people. Riding these waves of demographic changes, e.g. moving to mobile/tablets (app stores), or coming online in the first place (India), just makes success that much easier.
The opposite is true too. I cringe when an entrepreneur tells me they're going into news or music. It's a little different since people continue to consume those things, but in general those industries are in turmoil and shedding people and market cap instead of growing substantially. It feels like voluntarily running into a burning building and trying to find some valuables and get out before the whole thing collapses. Whereas India feels more like going to a huge concert and selling water.