From WoWWiki

This article or section needs to be cleaned up to a higher standard of quality. This may include editing to correct spelling and grammar, rewriting sections to ensure they are clear and concise, and wikifying. Reason: While this info may be accurate, it's not presented well (how can you tell what the spell level is, for instance). It's also fairly irrelevent since the 2.0 patch, since you'll automatically put the best buff possible on the target. When I saw the name, I expected to also find out the maximum number of buffs/debuffs on a given target. I didn't expect this info at all.

There is a lower bound on all buffs, the target cannot receive a buff with a spell level greater than their level + 10. EG: a level 10 cannot receive a level 21 buff from a player spell.

However, the game does scale all spells cast on friendly targets. This means you can cast Mark of the Wild on someone 60 levels lower than you, and it will automatically scale to the highest rank of MoTW for their level.