Considering Lebron is better than ever and he's 28 now, he probably won't start to slip at all until 30-31, but maintaining his current level after that is a huge stretch. That's usually when players start to slip a bit from their prime level anyway, but still remain close to it for at least a couple of years.
It'll be interesting to see how Lebron ages compared to Kobe. Kobe has always looked much more fluid than Lebron and been more skilled, plus he's always been a hell of lighter, he maxed out at 225-230 a couple of seasons('03 and '05) but has been in the 210-215 range for the most part and dropped down to 200 in '08 to take pressure off his knees. Lebron was heavier at 18(245) than Kobe ever was and has been 265-270 for about 5 seasons now. But Kobe had 2 knee surgeries by age 28 while Lebron has shockingly been injury free so that works in his favor.