I wondered this, too. My dd is 2.5, and about 3 months ago got a bad headcold...so I gave her a pillow one night to keep her head elevated so she could breathe better. 2 nights later I went to remove it and she went bananas!!!! "I need my pillow, Mama!!!" I called my sis and my SIL, asking if this was ok, (both have way more experience than me!) and they both thought it was. I wouldn't have offered it until she was 3....but what's done is done...she's happy and she's not giving that pillow up!!
But.....although she "goes" to sleep with her head on it, when we check her before we go to bed, or at any time during the night, she is not on that pillow at all...she scoots down so her head is on the mattress and the pillow is up at the top, with nobody on it!
I *thought* I had read that 3 yrs was the time to offer, but I can't be sure....this will be an interesting thread!!