The problem occurred when you wrote x=0 and then int([x[age]]). When you declared x=0, it created an int variable which value is 0. Then again when you wrote int([x[age]]), compiler assumed it as it an array but you didn’t declare it as an array.
The whole program would be like this
name = raw_input("What's your name? ")
age = raw_input ("how old are you? ")
x = 0
x = int(age)
twentyone = 21 - x
print "Hello, " + name+ " you will be 21 in: " + str(twentyone) + " years."
It should work fine now.