Why does @click='setAge(this.age - 1)' throw an isNaN error?

Question

typescript

@click='setAge(this.age - 1)'

export default class ModalSelectAge extends Vue implements IUser
{
    @namespace(USER).Getter
    age!:  number;
    @namespace(USER).Action
    setAge!: (age: number) => void;

setAge({commit}, data: number): void {
        console.trace({data});

The question of why the action was entered at all was asked earlier, the validation of the number data type was missed and NaN was placed in the storage.
But why can't you use a setter by passing the getter value + 1 to the parameter?