You are getting the error because of the following line:-
int([x[age1]])
So, to get rid of the error you need to convert the int to a string for the output see the code below:-
print "Hi, " + name1+ " you will be 21 in: " + str(twentyone) + " years."
The complete code is as follows:-
name1 = raw_input("What's your name? ")
age1 = raw_input ("how old are you? ")
x = 0 x = int(age1)
twentyone = 21 - x
print "Hi, " + name1+ " you will be 21 in: " + str(twentyone) + " years."
To know more about this you can have a look at the following video tutorial:-