oomis wrote: “The Torah refer to man as an eitz hasadeh (hope I got that right), and just as a fruit tree halachically is not shorn of it fruit for three years, there arose a minhag therefore, to not cut the hair until age three.”
Here is the reference:
http://www.hebrewbooks.org/pdfpager.aspx?req=14573&st=&pgnum=377
The Medrash states that the posuk of allowing the trees to be unshorn for 3 years refers to a child. This is very close proximity to the posuk that prohibits the cutting of peyos. This juxtaposition is interpreted to suggest that the chinuch for peyos be at age three, and that the fourth year will be marked by the fruit being “kodesh hilulim laHashem”, holy to HKB”H.