He does happen to be 17, for what it's worth. He's glad to have someone to team up with and gives a firm nod before springing into action.

"Hoo yeah! Glad to join the battle party, it's just my jam!" Considering that these are undead monsters that can't be reasoned with, Finn doesn't have anything holding him back in a battle. As Utena's foe falls back into the water, Finn springs forward and drives his sword into the creature, most likely finishing it off. He then pulls the sword back up and dashes forward to try to cut through some of the incoming revenants. But there's a lot to keep track of!