elindholm
edited for content
Name a player who has led a championship contending team and dominated past 35 years old.
Kareem Abdul-Jabbar averaged 22 and 8 for the 1985 title-winning Lakers. He was 38 by the start of the playoffs. One could argue that the team was "led" by Magic Johnson, but Abdul-Jabbar was their top scorer and rebounder, in both the regular season and the playoffs.
Besides, the fact that something hasn't been done before is not much of an indication that James can't do it.