Well... there are 18 cards out at the table... and one of them for sure is an ace because you hold it... there are 52 cards in the deck and 4 of them are aces.. so its makes pretty good sense because if you figure there are 18 cards out.. almost every time someone is going to have an ace and then about 3/4 times there are going to be two aces... i would say 1.5/4 there will be three aces and .5/4 there will be four aces out.. but thats just some guesses off the top of my head not looking at odds...
More important than that is, what are the chances somebody has an Ace with a higher kicker. Wikipedia has a real good table showing the odds for every kicker and every number of players at the table.