The trouble with any long-running game is that it gets top-heavy. This is especially a problem in games such as Final Fantasy XI, where you simply have to group to accomplish most things in the game, but it's really a problem even in the most solo-friendly game with an expansion or two. After all, more often than not there's an ever-raising level cap, gear curve, and expected knowledge about the game, not to mention stretches of previously endgame content that's no longer relevant. When A Casual Stroll to Mordor asks if it's too late to start playing Lord of the Rings Online, the question could be extended to many other games.
Of course, it's probably not exceptionally surprising that the conclusion is "no," duly pointing out that the only time it's technically too late is when the game is shutting down soon. There are also tips about making the areas at lower levels less of a ghost town, which largely boil down to being proactive. People will almost always have alts or restarted characters, and actively trying to get your foot in the door can make up for a lot of inexperience. It's as true in Lord of the Rings Online as it is everywhere: putting forth the effort makes a lot of difference, even if it won't close the expanded level gap by itself.