Question 900867
I'll say:
A's age  = {{{ a }}}
B's age  = {{{ b }}}
------------------
I will also say that the problem starts 
off talking about A
-------------
" When I had the age you have " means 
when A's age  = {{{ b }}}
----------------------
How old was B then?
The difference in their ages never changes, so
B was {{{ b - ( a  - b ) = 2b - a }}} 
-----------------------------
it says A is presently twice that age, so
(1) {{{ a = 2*( 2b - a ) }}}
---------------------
" and when you'll be my age " means B's 
age  = {{{ a }}}
At that time, A's age will be {{{ a + ( a - b ) = 2a - b }}}
---------------------
The sum of these ages is:
(2) {{{ 2a - b + a = 72 }}}
----------------------
(1) and (2) have 2 unknowns, so it's solvable
(1) {{{ a = 2*( 2b - a ) }}}
(1) {{{ a = 4b - 2a }}}
(1) {{{ 3a = 4b }}}
(1) {{{ b = ( 3/4 )*a }}}
--------------------
(2) {{{ 2a - b + a = 72 }}}
(2) {{{ 3a - b = 72 }}}
Substitute (1) into (2)
(2) {{{ 3a - (3/4)*a = 72 }}}
(2) {{{ 12a - 3a = 288 }}}
(2) {{{ 9a = 288 }}}
(2) {{{ a = 32 }}}
and, since
(1) {{{ b = ( 3/4 )*a }}}
(1) {{{ b = ( 3/4 )*32 }}}
(1) {{{ b = 24 }}}
-----------------
A is 32
B is 24 
---------
check:
" when i had the age you have "
( When A was 24 )
How old was B then?
B was {{{ 24 - ( 32 - 24 ) = 24 - 8 }}}
{{{ 24 - 8 = 16 }}}
------------------
A is twice that age, or {{{ A = 32 }}}
correct
------------
When B will be 32, and A will be
{{{ 32 + ( 32 - 24 ) = 40 }}}, the sum 
of their age will be 72
{{{ 32 + 40 = 72 }}}
correct again
-------------
Hope it's not confusing