The same guys who did this blog post (BackBlaze) did another on failure rates based on age:
Their basic findings was that the first 18 months has a 5.1% annual failure rate (infant mortality), the following 18 months has a 1.4% annual failure rate (random failures), and 36+ months shows a 11.8% annual failure rate (old age). They had 4 years of data to use.
If you look at their charts, they're not completely flat/straight lines, but it's surprising how straight they are, and how sudden the inflection points are.
Using absolute survival figures, roughly (because I'm reading off a graph) 95% survive one year, 92% survive two years, 90% survive 3 years, 78% survive four years, and their extrapolation is that you hit 50% at about 6 years.