First of all, emulating the speach of an 18 month-old is not that hard. But, extrapolating that to a 5-year-old and then further is a bit too presumptuious.
I can build an emulation of a 9 month child, although my Puke-A-Tron still has a few glitches in it.
Did Turing specify what kind of human was to be emulated? The person on the other side could possibly mistake some of the current chat bots for skiztofranic humans.
However, they will never be able to emulate my bad spelling. That should be the *real* test.