I can't remeber witch but I'm almost sure that in a episode in the middle oder in the end of season 4 Booth says he's 7.. So I think he turned 7 in season 4 but he was 6 in "The finger in the nest"
posted Vor mehr als einem Jahr
And Booth says he's 4 in season 1 and if every season is ca. 1 Jahr then he probably turns 7 in the end of season 4 ;)