That cannot be because for any two people, the ratio of the older's age to the
younger's age shrinks, and never grows. So if the Mother has already been twice
her son's age, she will never be three times in the future.  However, she could
have been three times as old as her son in the past, just never again in the
future. 

Suppose the Mother is 50 and her son is 26.  Then two years ago the Mother was
48 and the son was 24.  So the Mother was twice the son's age 2 years ago.
That fits the problem's specification.  Now 14 years AGO the Mother was 36
and the son was 12,  So the Mother was three times as old as her son was 14
years ago.  But see? That's in the past, not in the future.  You asked when in
the future.  The problem is meaningless as you have stated it.  I suspect
there there is something you didn't state about the problem, unless "no
solution" could be accepted as the correct answer.

Edwin