I seriously doubt James would be a bench player at age 38. The most likely comparison based on body size is Karl Malone and he was still starting and playing over 30 mpg at 39 (and started until he retired). Kareem, Garnett, Jordan, etc. were all starting until they retired older than 38 though did scale back the minutes a lot (well not MJ he played 37 mpg at age 39. Heck Dirk just finished this past year starting and playing 26 mpg at age 38
You can't look at just age. You also have to look at minutes played which directly results in wear and tear. Lebron started young and his playoff experience (2nd in minutes to Tim Duncan) means more wear and tear than the average 32 year old. Lebron is 32 and has already played 50,000 minutes between the regular season and the playoffs. He's averaging 3,300 to 3,500 per year. He's already played more minutes than Jordan did in his whole career.
Karl Malone played until he was 40 (he was a physical freak like Lebron) but played a bit less than 63,000 total minutes. 'Bron will be 36/37 when he hits that mark. Dirk has a bit over 54,000 minutes. KG is somewhere north of 55k. Pierce is at 52k. All of those guys have had a noticeable decline in play.
To put it another way, if Dirk retires then Lebron will have more minutes played than any other active NBA player. Now I wouldn't put anything past that freak-of-nature. Lebron is one of a kind. But at some point the wear and tear will take its toll. If Lebron is still playing at 38, he'll likely be setting the record for the most minutes played in the NBA (combining regular season and playoffs). Will he start at that age? I'm not so sure if he'll still be playing.