According to the question

We use the formula,

Value of new observations= a - nb

[here, Average decreases, hence '-' sign (a-nb) has been taken.]

Here, n=24, a= 12+13+14=39 yr, b= 2 months= $\frac{2}{12}$ yr

Total age of 3 males = a-nb

= 39-24×$\frac{2}{12}$ 

= 39-4= 35 yr

Average age of 3 males= $\frac{35}{3}= 11\frac{2}{3}$ yr

Hence, Option D is correct.