Kobe retired at 38 year old, he was still a star though he was already struggling at that time, but Lebron is different, he is healthy so he could last at 40 years old or even more, who knows, his conditioning is extra ordinary, he spent a lot of money for that and he wants to prove something.
From 2012 onwards, Lakers is one of the underperformed team in the NBA. Kobe Bryant suffered major injuries that affects his overall performance. If my calculation is correct he was 33-36 years old at that time.
If Lebron will make it possible not to suffer injuries, he can even play at 40s in a strong built and established a new record of the most active age player in the NBA which I think being hold now by Vince Carter at 43 years old.