I wasn't there (at court) so I have no clue what else was said but had to go back and re read his testimony. I too thought RB said he took DM in from age 15-17 years old as well. It does sound like RB is saying that he didn't have much contact with DM in his younger years but DM lived with him for 2 years.
From Adam Carter
Burns says he knew Millard "sporadically" as he was growing up.
Adam Carter
Millard's father Wayne Millard and Millard's mother Madeleine Burns separated for a time, Burns says. For "altruistic reasons," he says, he watched Millard from ages 15 to 17
BTW, I'm not arguing but it seems maybe, we home viewers missed some information?