I just returned from a 7 night Western Magic Cruise.
Tritons on the wonder is a replica of Lumiere's on the magic.
In lumieres a lot of people were wearing jackets.
I was not.(I am also 12 years old but.....) If you do buy a jacket it would be for before dinner when you get your pictures taken. During dinner you would probably take it off so you do not spill. thats what most people were doing at least.
I just wore a Shirt + Tie and looked perfectly fine.