... or use a constant current regulator.

Leds in series with a current sense resistor at the bottom. Regulator will up the supply voltage until enough current flows through the resistor to generate the devices feedback voltage. As the Leds age they go dim because their resistance increases. No problem, regulator will up the voltage to compensate giving you a constant brightness.
Connect the enable function to a PWM output, and hey presto PWM dimming!