On average, mobile phones are replaced every 24 months. So trailing 24m sales are a pretty good proxy for the install base. On that basis, at the end of 2012 the Android install base was about 675m, and the iPhone base was about 230m. (Apple discloses these numbers so you can just work them out: Google gives Android activations numbers with sufficient frequency to work out a run rate, to which you have to add an estimate of Chinese sales, since they're not included in the activation numbers.)
The interesting thing is to think abut how those bases have grown. At the end of 2010 (24 months earlier), Android had sold 55-60m units since launch about a year earlier, and the iPhone had sold 72.7m units in the previous 24 months.
Applying a little arithmetic, we see that a little over 90% of the current Android base got its first Android in the last 2 years, and 68% of the iPhone base its first iPhone.
Of course, this ignores switching between the platforms, for which there is no good data, but a lot of it will net out. It also ignores people buying a new phone every year - but also people who are still on 3 or 4 year old phones - but these are buried in the average. The number of people selling their iPhone to buy a new one every year is the big variable here.
To put that another way, about 90% of current Android users are on their first Android, as are 70% of iPhone users. It'll be interesting to see what their second purchase is.