The following was developed as a mental age assessment by the School of Psychiatry at Harvard:

Take your time and see if you can read each line aloud without a mistake.
 
The average person over 45 years of age cannot do it!
  
  1.This is this cat.
     2. This is is cat.
        3. This is how cat.
           4. This is to cat.
            5. This is keep cat.
               6. This is an cat.
                  7. This is old cat.
                        8. This is fart cat.
                           9. This is busy cat.
                               10. This is for cat.
                                    11. This is forty cat.
                                          12. This is seconds cat.
  
Now go back and read the third word in each line from the top down.