16 and 23

Stephanie seems to be good at counting... As she is my oldest child, I am not sure what the skill level should be, but I am satisfied with her abilities.  She understands how to count in the higher numbers, though sometimes needs prompting with the first number.  For instance, once she'll get to "twenty nine, twenty ten", I say, "twenty ten is called thirty", and then she starts in on "thirty one, thirty two..."  except for the numbers 16 and 23.  They are missing, and have been for a while.  Yesterday she asked me what 10 plus 10 more was.  I said 20.  She said 21.  I said, no 20.  She insisted it was 21.  I told her to count it out, as I had already realized she was asking me 10 plus 10 because she was counting her fingers and toes.  She had already done the counting, and then was asking me to see if I knew the answer.  So she counts out loud, got to fifteen, then jumped to seventeen, and thus arrived at 10 plus 10 is 21.  She used to combine the words "fifteen and sixteen" to make a number like "fixteen", but she has apparently more recently decided to settle on keeping 15 and wholly ignoring 16.  I do correct her from time to time, but I don't make a big deal out of it, as I think it will work itself out. 23 is also missing, which is interesting because 33 and 43 and 53 are not missing.  But ask her to count to 25, and 23 is never there.  As a person that enjoys math, I can leave or take 23, as I don't find prime numbers that interesting, but I do really miss 16.  16 is a great number, it even has a square root, and not all numbers can claim that honor.  I am certain we will get them both back with time, but in the meantime, Stephanie can say with confidence that she has 21 fingers and toes.