Visit almost any MMORPG that's been around for a year or longer, and you're bound to find that the game is top-heavy. A heavy focus on end-game content, whether it's raiding, instances, or other max-level content, has led many community populations to become heavily concentrated with characters at maximum level; for example, almost a full 50% of active World of Warcraft characters on US servers are level 80 based on Warcraft census data. While lower-level characters do exist in the form of new players and alts, and these games usually have a reasonable amount of low-level group content created during their development and growth, that content often becomes abandoned and neglected by players on their rush to join the masses on top.
There are plenty of players who want to experience that content, of course, even if they have experienced it before. Unfortunately, the numbers of those players are often scattered widely between time zones and servers, making it difficult to find the prerequisite number of people to fill a group - much less a good group composition. For the lucky people who have managed to hit the right compromise of factors, low-level group content still gets its time in the spotlight. For those who aren't so lucky, other solutions have to be found.
The most typical solution, and coincidentally the most recommended solution, is to have a high-level friend run through the content. Simple, right? Just grab a buddy who's high enough to run through the content solo, and then tag along while they do the dirty work. Don't have a high level buddy? Pay off a random high level, mercenary style, to be your friend until the instance is done. Join a guild, and hope their members are kind enough to new or low level players to help you through - although some guilds are actually so inclined to host regular "lowbie" runs to help lower level characters through group content. Although players may not get to experience the content the way it was intended to be experienced, and may not learn anything about group dynamics, they will, at least, get to experience the content, and gain experience, loot, quest progression, and exposure to story and lore they couldn't see alone.
Some games, like EverQuest, have smartly picked up on the fact that friends want to help friends in a social game, and have introduced mentoring/apprenticeship systems. These systems allow higher level characters to "level down" temporarily to match that of their lower level friends. This saves the step of having a friend power you through content, or forcing them to reroll and play a new character at your side all the time. These systems are great for questing, or for players who have several friends that can put together a proper lower level group, but for players who only have one friend to play with, that still leaves a need for more lower level characters to join in.
What players want - and games need - are compelling reasons to re-run older content. Consider the skirmish system coming up for Lord of the Rings Online's Siege of Mirkwood expansion. The new skirmishes, which act as dynamic instances, are scalable based on difficulty, group size, and player level. Although not a lot of details are out about this system yet, we're left to assume that the level of rewards will be based on the level of the skirmish setting, and that the level setting assumes all three group members are of that level - meaning that a lower level character will not survive a high level skirmish, and a high level character will find no reward in a low level skirmish. What the skirmish system does provide, however, is a scaling instance system that invites players to revisit content. A few tweaks to average out the difficulty based on average player level could provide a balanced way to encourage high level players to play along lower level players.
Or imagine that a game held a mentoring system, tailoring players down to the level-appropriate stats, but instead of just the feel good of helping someone through a run, players were actually rewarded with a special currency that allowed them to purchase unique items only obtainable by helping low-level players? To prevent abuse, these tailored groups could require that at least one player not be leveled down by the mentoring system for the instance - meaning that at least one character is always benefiting from the system. Sure, we as players should help others out of the kindness we naturally possess, but let's face it - most players need a reward carrot dangled in front of them to encourage them to do anything.
But what if the problem is visiting the content in the first place? Group content has become increasingly optional, as has the loot rewarded from it. Many games have tuned leveling so that a player can easily solo via grinding and questing, relying only on quest rewards and items purchased from other players, until they reach max level. The game suddenly turns to group play, and many players have had no experience yet in groups that they do miserably in them.
Games could gate characters through group-required areas such as dungeons or instances, and prevent higher level players coming through (unless under a mentoring system.) Considering a fictional scenario, in which players are presented with a story instance at level 20 - but instead of skipping past it to continue leveling, players must complete the instance in a level-appropriate group. Equipment dropped here could be worth a substantial number of levels - not only the best until you reach the next dungeon, but the best by a wide margin. Without completing the instance, characters will be held at a gated level, with no new quests to find, until the instance is complete. Of course, this doesn't work if the game is too top-heavy and the lower level areas are ghost towns.
I don't personally advocate forcing group content on players, but the content should be compelling enough to revisit - and players need to be willing to do so. Striking a balance between end game content and keeping lower level content interesting and populated is nowhere near as easy as any of the methods I've brought up. Ultimately, it's up to player communities to encourage participation in content; go beyond complaining that a group can't be found and be the leader to put together groups and community efforts to ensure that all players have a chance to experience content "as intended" - and not feel forced to rush to max level.