The issue is in the line:
int([x[age1]])
The solution you want is:
x = int(age1)
Convert the int to a string for the output.
print "Hi, " + name1+ " you will be 21 in: " + str(twentyone) + " years."
This is the complete script:
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."
I hope this helps.