The average age of four boys A, B, C and D is 7 years and the average age of A, B, D and E is 8 years. C is 12 years old. The age of E is (in years) :

16 years

The average age of four boys A, B, C and D = 7 years

A + B + C + D = 28 years

 C = 12 years 

so,  A + B + 12 + D = 28 years

A + B+ D = 16 years

 The average age of A, B, D and  E = 8 years 

Now  A + B + D + E = 32 years

 A + B+ D = 16 years

E = 32 - 16 = 16 years.