"Your IQ is, basically, your mental age divided by your actual age, multiplied by 100", it says. That means:
IQ = MA/AA * 100
To calculate your mental age, you have to change the equation:
MA = IQ/100 * AA
In your case:
MA = 124/100 * 17 = 21,08
Edit: I just saw you already did the calucation on an earlier page I did not read.