My age is

$\text{G=age of grandson}$

$\text{S=age of son}=7G$

$\text{M=my age}=12G$

The sum of the ages is,

$G+S+M=120$

Substituting, we get

$G+7G+12G=120$

$G=6$

It follows that

$S=7G=7(6)=42$

and

$M=12G=12(6)=72$

Solution: I am 72 years old.

Let m be my age in years. If s is my son's age in years, then my son is 52s weeks old. If g is my grandson's age in years, then my grandson is 365g days old. Thus,

365g = 52s.

Since my grandson is 12g months old,

12g = m.

Since my grandson, my son and I together are 120 years,

g + s + m = 120.

The above system of 3 equations in 3 unknowns (g, s and m) can be solved as follows.

m / 12 + 365 m / (52 x 12) + m = 120 or

52 m + 365 m + 624 m = 624 x 120 or

m = 624 x 120 / 1041 = 72.

So, I am 72 years old.

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