However, you're really going to need to explain what you mean by this. Are the other character's all tenth level, or are they just fifth level? Or are they somewhere in-between? Basically, your comment tells me everything I need to know about your Rogue, but not the status of your Fighter and Barbarian.
And when you say old rules, do you mean, like, 3.5 Multiclassing? Because this is the first I'm hearing of variant rules for MCing in 5e. At least official (or previously official) ones.
I think dnd4vr is talking about
really old multiclassing, 1E-style. In modern versions of the game, it would be known as "gestalt" multiclassing.
*dons grognard hat*
The way it worked back in the day was, if you wanted to multiclass, you did it at character creation. From then on, all earned XP was split evenly between your classes. You shared the abilities of all the classes, and averaged their hit points.
Thus, let's say each PC currently has 20,000 XP. For a regular PC, that would put them at 6th level, close to 7th. The rogue/fighter would split those XP between classes: 10,000 to the rogue half, and 10,000 to the fighter half, which means level 5 in both classes*. The character has all the abilities of a rogue 5 and a fighter 5; hit points halfway between fighter 5 and rogue 5; and the proficiency bonus of a 5th-level character.
If that seems brokenly powerful to you... well, that's because it is. Multiclassing was insanely good in AD&D, which is probably why the system was completely scrapped for 3E and replaced with a streamlined version of the AD&D dual-classing mechanic.
[SIZE=-2]*A 5E character multiclassing this way will always have the same level in all classes. In 1E and 2E, each class had its own XP table, so a multiclassed PC's levels would not always be in sync.[/SIZE]