My DS is 6.5 and his are almost completely gone. You have to really strain to see them, and be in really good light. When he was a baby, it looked like the whole bottom part of his back was bruised. My MW is the one who told me what they were, because I had no idea. DH is Mexican-American and I am glaringly Caucasian (Irish-German descent) so I was unprepared for Mongolian spots. My XDH, father of my older three kids, is white like me, Scottish-American descent, so our kids had "stork-bites" and "angel-kisses," but no bruise-colored spots.
My DD who is almost 4 is considerably darker than DS, but she did not have Mongolian spots, nor did Lil'Man. The baby is pretty fair, though, with dark red hair and hazel eyes, not Latino-looking at all (though his daddy is the same).