Let grandfather’s age be G
Then Alex’ is 
Let “a few years” be y. Then:     Let “another few years,” or years after this year, be z. Then: 
                                                                    We then get: 

                     With , ONLY MULTIPLES of 3 can be substituted for y in order to get INTEGER-VALUES for z

Therefore, if y = 3, then 
This gives us: G, or the grandfather’s age as: 24(3), or 9(8) = 72.

With ONLY MULTIPLES of 3 that can be substituted for y, we substitute 6 for y to get: 
This gives us: G, or the grandfather’s age as: 24(6), or 9(16) = 144

As it’s HIGHLY UNLIKELY that the grandfather is 144 years old, no LARGER multiples of 3 need to be tried/substituted. 

Therefore, the more REALISTIC age for him is as stated above, 72.