When Arthur was fiddling with the gun, he had unintentionally set the age regression to 'Regress by the minute'.
That meant that , Arthur was gonna regress in age - by one year, every minute.
As he was getting his food at the local department store, he suddenly seemed to regress one whole year, in just about a minute or two.
Then another. And another. And another.
As he was walking home, it took less than 5 minutes for him to regress more.
5 minutes later, he was now 22 years old.
He was surprised. Was he shrinking? Was he losing weight.
Was he getting weaker.
Was there something wrong with his age?
He seemed to be shrinking smaller and smaller and smaller and smaller and smaller and smaller than ever before.