Buff limit

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.