Does (and when how) the lifetime of a LED depend on the PWM frequency
Let me open my trusty MIL-HDBK-217F and see what it says about LEDs and their longevity: -



The main factor affecting the failure rate per million hours is temperature. 

Of interest, if I read the next section about laser diodes they do take into account duty cycle pulsing but their conclusion (on page 6-21) is that at 50:50 duty cycle the failure rate for laser diodes is about 25% of that when continually driven.

They also conclude (on page 6-22) that if you operate a laser diode at a light output power of 50% of its rating it will last ten times longer than operating it at 95% of it's rated output power.