One man speaking to other:
When you were born I was 20 years old. Today I am twice the age you were when I was the age you are now.
What are the ages of both of them?
80 and 60
I think it's helpful to break up and reword the problem: "I am 20 years older than you." "When I was your current age, you were (of course) 20 years younger than me." "I am now twice the age you were at that time."
And I don't really know how I got from that, to 80. But it works:
Man A is 20 years older than man B. When man A is 80, man B is 60. When man A was 60, man B was 40. "When I was your current age (60), you were 20 years younger than me (40)." "I am now twice the age you were at that time (80)."