Launch risk does seem to be a function of experience -- the Russians have been flying pretty much the same rocket for nearly 50 years. That provides a lot of opportunity for incremental improvement. Combine this with early Russian practice (based largely on lack of sophisticated testing equipment) of testing rocket designs through live flight.
But risk in general is also a function of use and operational time. This has been borne out in many contexts, including both space flight and aviation. Systems degrade in nondeterministic ways over time, increasing failure risk. Even very minor variations in design -- a few mm of protrusion in a fuel-oil heat exchanger in the Boeing 777, implicated in the British Airways 38 Heathrow crash, given a specific set of circumstances in in-flight ambient temperatures and engine throttle settings resulting in ice-induced fuel starvation -- can have profound impacts.
But risk in general is also a function of use and operational time. This has been borne out in many contexts, including both space flight and aviation. Systems degrade in nondeterministic ways over time, increasing failure risk. Even very minor variations in design -- a few mm of protrusion in a fuel-oil heat exchanger in the Boeing 777, implicated in the British Airways 38 Heathrow crash, given a specific set of circumstances in in-flight ambient temperatures and engine throttle settings resulting in ice-induced fuel starvation -- can have profound impacts.
https://en.wikipedia.org/wiki/British_Airways_Flight_38
It'll be interesting to see how re-use, risk, and cost play out with Space-X.