Has LBJ slowed down? Of course he has. He has lost a bit of athleticism, which shouldn't be much of a surprise considering his age, coupled with the wear and tear his body has endured.
It's tough to predict how his game will age. There are few players to compare him to throughout NBA history. For chrissakes, he's only missed roughly 30 games throughout his entire career due to injuries. Few players take care of their body like James. He is truly a perfectionist in almost every sense of the world. He literally lives to get better at basketball each day. The man needs no motivation, which is rare in this day and age when money is thrown around like it grows on trees.
That one day where we see a decline in his on-court ability shouldn't necessarily be feared. he could easily be moved to the 4, creating a mismatch for almost every team in the NBA. His strength and speed, coupled with his intellect, will make him as close to an impossible guard at the 4 this league has seen. Consider that for a second. In LBJ's twilight years, he could still be shaping how the game is played. We've all bitched how LBJ has never established a consistent jumper but when player's age, usually their jump shot is the first to suffer. The rest of their game tends to fall off after that, which could be due to a myriad of reasons not necessarily physically-related (loss of purpose/identity, will to perform at a high level, etc.) With LBJ, he's perfected almost every other intangible associated with being a complete NBA player. His game is set up to age gracefully.