The solo tag means, it's an individual task or quest, and not that it was designed to be done solo, but that it is only given to a single player at a time, as compared to shared group tasks which are given to the whole group. As you noted, the level is generally the minimum known level to request the quest or task.
I believe the reason for this is, as the game progresses, any particular quest or task in general, becomes easier to do at a given level, due to a multitude of factors. As an example, when OoW came out, getting the signets even at level 70, was a challenge for a group, unless they were tacvi geared. Now-a-days (although not necessary to do), level 70 defiant gear is much better than tacvi gear overall, and all of the signets can be molo'd at 70, if not lower levels, and possibly even soloed.
Even at a given time, alot of the ability to successfully complete a quest or task, depends on the number, tactics, and gear of the players involved to determine the minimum level it could be accomplished.
Along the same lines, the EQ1 con system isn't all that specific. A dark blue raid con can be much more difficult, or much easier than a yellow non-raid con. Thus, the key to determining if a particular quest is doable is noting when and with that level, gear, and tactics someone else had completed the task. If such information is not available, you can generally infer the requirements from the listed mobs that need killed which will usually have some info on their difficulty. If not, you'll just have to try it out and see.
It's just the nature (design) of EQ1. Many other games are much better at signifying the requirements needed to complete at a task a given level, but even in those games there is some variability depending on classes, tactics, etc.
Personally, I don't really look at quest level, unless it's listed way higher than my current level. I look for what zones I have to kill in, and if any nameds, and how tough they are described, if I'm unsure of their difficulty. And then the specifics of the mobs, if they seem like they might require tactics different than my usual for the toon, and if it seems doable.
Yther Ore.