What do either of those things have to do with being a ‘grown up’? Your original assertion was that wearing a backpack in any situation was ‘childish’ but now you’re adding extra qualifiers in an attempt to strengthen your argument.

’Appropriate’ is pretty subjective. The point is that you are entitled to your opinion on wearing backpacks, but if it’s allowed by the FOM and/or uniform policy then your feelings on the matter are no more or less valid than anyone else’s.

For the record I think it ‘looks’ kinda dumb for a pilot to be wearing a backpack walking through the terminal, but it’s their perogative.