Lebron is old in basketball standards, and really 35 is pretty old for any sport. Lebron has already shown signs of his age and slowing down, though sure it can be hard to tell considering he's still one of the top players in the world. That said Lebron has only a couple years left. No one is a super star in to their 40's in basketball. I do think he's got 2-3 years of greatness left however.
That's right, he's got 2-3 years more to dominate the league. After that, changes of him slowing down will become very obvious. But, I guess he won't retire until he's 40+, he's been looking forward to play with hi son in the NBA and I guess the NBA would be allowing that to happen. It's all business after all, LeBron is a big name in the league, so why not?.
Though just like what you said, no players in their 40's is a superstar in basketball, because they become a living legend.