Charles Barkley does not rank LeBron James as a top 7 player, in part, because LeBron had a "head start" in his first 13 seasons.
Although at first blush that comment makes no sense (13 seasons is 13 seasons, you are evaluating the same thing) a friend said he thought Barkley's point was that LeBron got to play his first 13 seasons at ages 19-31 (whereas Barkley and Jordan, for example, got to play theirs at ages 21-33 or 22-34).
So let's put it to the test and review the issue of what sort of Win Shares do guys put up ages 19-21 versus ages 30-32 and 31-33.
1) 53 guys have put up at least 10 WS by the time they reached their age 21 season (which basketball-reference.com defines as turning 21 before February 1 of the season). http://bkref.com/tiny/vdGNF LeBron is the top at 35.6, the lowest guy on the top 10 is Andre Drummond at 22.0. The median of the top 10 is 25 WS. The median of the 53 guy list is Brook Lopez at 13.7 WS.
2) There are 259 guys who have put up at least 10 WS ages 31-33: http://bkref.com/tiny/Op77U
Karl Malone is #1 at 45.7WS. The 10th ranked guy is Artis Gilmore at 35.4. The median of the top 10 is 40.1 WS. The overall median of the 259 guy list is Jerry Sloan at 14.6 WS.
3) Moving on to ages 32-34. There are 186 guys with at least 10 WS. http://bkref.com/tiny/HAowU The top guy is Michael Jordan at 54.5WS and #10 is John Havlicek at 31.5 WS. The median of the top 10 is 35.8 WS and the median of the entire list is 17 WS.
Ergo, what we see is that there is no evidence that it is better to play your first 3 seasons at ages 19-21 or even 18-21 than it is to play seasons at ages 31-33 or 32-34. If anything, the statistics indicate that you will generally play better at ages 31-33 and 32-34 (and there is generally no significant difference whether it is 31-33 or 32-34).
If you look at the age 31-33 list, in particular, you will see that LeBron's early career 35.6 WS gave him very little, if any, advantage on the 11th-12th-13th seasons of guys with whom he is generally compared. There is either no difference or a very slight difference that cuts both ways depending upon the player.
The greatest indictment of Barkley's claim (if it can fairly be said to be his claim) is that Michael Jordan (to whom James is generally compared) at an older age was better than the 19-21 year old LeBron James, and using the 32-34 age period, the old Jordan was better than the young LeBron by over 6 WS per season. So, if you take two all-time greats and play one at age 19-21 and one at 32-34, they will both lead their age groups in WS, but the one who plays 32-34 will be roughly 50% better!
CONCLUSION -- as a general rule, it is better for your stats to play your first thirteen years in the NBA at ages 21-33 or 22-34 rather than ages 19-31.