What does Bryon mean when he says "that was then, this is now"?

In Chapter 4, Bryon and Mark are reminiscing about playing as children and getting into trouble with their friends when they were young. Mark then asks Bryon if he ever gets the feeling that things are coming to an end because something new is about to begin. Bryon tells Mark that he understands how he is feeling. Mark then mentions how they used to be very close with their other friends, but recently things have begun to change. They don't get to see their friends anymore because everyone is in a relationship or doing their own thing. Bryon agrees with Mark that things have changed and says that it is kind of a good thing. Mark then sighs and comments, "but there's a difference. I wonder what the difference is?" (Hinton 69). Bryon responds by saying,

"The difference is...that was then, this is now" (Hinton 69).

Bryon essentially means that as time progresses, people change and grow apart from one another. Bryon realizes that their childhood has come to an end and that the present stage of their lives is not the same as it once was. He is saying that the past is the past, and one must accept the ever-changing course of life.