Think it depends on the child n their understanding really. I have been using one since DS was 2.9, later than most. Bought it months earlier but he just didn't grasp it til then despite being really bright.
U could suck it n see n if it doesn't work yet try again each month until it does?
Mine understood it from that age. If they've got the receptive language to understand "blue moon, sleep time", and "yellow sun, getting up time" then it should work.
Of course, understanding is one thing, being prepared to play along is another... We were boringly consistent about "blue moon, time for sleep" and it worked very well both for early mornings and nap times (which morphed into quiet playing in their room until yellow sun times).