Why is Alex surprised to see that Dim and Billyboy are police officers?

Alex is surprised to see his old acquaintances as police officers for a couple reasons. First of all, he thinks that they are too young to have become cops, but Alex has forgotten that he was always the youngest of the gang, and that a lot of time has passed. Secondly, the sheer coincidence of the situation is too much for Alex, and he keeps expecting it all to be a joke. It is not.