Towing is the term used to describe the action of PC's of considerably varying levels, going into dungeons/areas where the lower level/s would stand no chance of survival on their own and/or are unable to productively contribute to a fight outside of simply healing.
It's generally a practice carried out by people wishing to gain the increased level of xp, due to the lower average party level. What it will in fact earn you, if you're caught by DM's doing it, is a trip to the halls and the subsequent loss of XP and gold that accompanies it.
When it comes to level differences, there is testing ongoing regarding this. However for the most part, outside areas are not even considered into the equation when we look at it.
Primarily our concern is dungeons.
That being said. It also has a lot to do with how it is being ran.
If the Lowbie is just running around with you invisibile and only coming up to heal you once in a while... chances are, you are going to get Pwned if a DM sees you.
However, if the lowbie is actually contributing. Actually out there fighting (and no i don't mean standing a mile away with a bow or sling just launching arrows) then chances are you will be watched, but left alone, and possibly even helped in some situations if a DM sees it.
The above is from a post a while ago regarding parties/allignments and towing.
The bottom line is: If you cannot contribute a reasonable amount to the fighting, then you shouldn't be there.
We're by no means saying that groups shouldn't consist of varying levels of PC's. What we're saying is that if you have low level PC's in the group, then the destination chosen should reflect the capabilities of those lower levels and not the ability of the higher levels to kill everything in sight.