I think LeBron will age very well because he can slide over to power forward and play there full time. Even with the loss of his explosive quickness, LeBron's skill-level (relative to a PF) will still make him a very potent player. The ball-handling, the secondary playmaking, the spot up shooting. He should still have a quickness advantage at PF for 5+ years to come to set up his dribble drive game. He has the post game to take advantage of smaller players if teams go small to matchup with him. He'll be a really big problem for years to come.
Remember when Magic Johnson came back to play in 1996 when he was 38 years old. He hadn't played in four and half years. Was badly conditioned. Overweight. Still, even at that age and with questionable conditioning, Magic Johnson was still a very good player. I think he had something like 15ppg, 7rpg and 6-7apg in low 30mpg (without training camp, joined mid-season for playoff push). You could visibly see how uncomfortable vast majority of PFs were trying to cover him out on the perimeter and the second a team switched a smaller defender onto him, Magic went down into the post and killed them there instead. Only a few teams had players with enough defensive range to cover Magic. So I think LeBron is going to be able to do something quite similar to that when his quickness goes as a player 36-39 year old player.