How to type an object with dynamic keys and different value types for each key?

I want to create a type for an object that allows me to use different keys depending on passed generic:

const withPrefix: User<"new"> = {
    new__firstname: "Tom",
    new__lastname: "Jennys",
    new__age: 15,
};

const withoutPrefix: User<undefined> = {
    firstname: "Tom",
    lastname: "Jennys",
    age: 15,
};

I created a utility type that returns the correct key depending on if there is a prefix or not:

type GetFullKey<
    TPrefix extends string | undefined,
    TKey extends string
> = TPrefix extends string ? `${TPrefix}__${TKey}` : TKey;

Then I use it like this to create a User object type:

export type User<TPrefix extends string | undefined> = {
    [key in GetFullKey<
        TPrefix,
        "firstname" | "lastname"
    >]: string;
    [key in GetFullKey<
        TPrefix,
        "age"
    >]: number;
};

It all works fine if I type each value as a string but when I want to type "age" as a number there is an error:

"A mapped type may not declare properties or methods."

How do I fix it?

You can't have multiple mappings in the same mapped type. You could redefine it as a single mapped type with a complex conditional type, or as an intersection of two mapped types, but the entire type definition feels a little strange.

Something like this feels more idiomatic:

type User = {
   firstname: string;
   lastname: string;
   age: number;
};

type Prefixed<T, P extends string | undefined = undefined> = {
   [K in Extract<keyof T, string> as P extends string ? `${P}__${K}` : K]: T[K]
};

const withPrefix: Prefixed<User, 'new'> = {
    new__firstname: "Tom",
    new__lastname: "Jennys",
    new__age: 15,
};

const withoutPrefix: Prefixed<User> = {
    firstname: "Tom",
    lastname: "Jennys",
    age: 15,
};

^{Playground link}