How to restrict possible value of index type?

We can declare index signature to force the properties of object to be of specific type such as below

const LOOKUP: {
  [key: string]: { name: string, age: number };
} = {
  isaac: { name: 'isaac', age: 20 },
  whatever: {name: 'whatever', age: 17}
};

However, I would like to restrict the possible values that goes as key, so below is my attempt

Attempt 1

type PossibleName = 'isaac' | 'john';

const LOOKUP: {
  [key: PossibleName]: { name: string, age: number };
} = {
  isaac: { name: 'isaac', age: 20 },
  whatever: {name: 'whatever', age: 17}
};

The attempt above hitting error below

An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.

Attempt 2

type PossibleName = 'isaac' | 'john';

const LOOKUP: {
  [key in PossibleName]: { name: string, age: number };
} = {
  isaac: { name: 'isaac', age: 20 },
};

And attempt 2 is hitting error below

Property 'john' is missing in type '{ isaac: { name: string; age: number; }; }' but required in type '{ isaac: { name: string; age: number; }; john: { name: string; age: number; }; }'

I can solve it by making the key optional such as below

type PossibleName = 'isaac' | 'john';

const LOOKUP: {
  [key in PossibleName]?: { name: string, age: number };
} = {
  isaac: undefined,
};

It does allow to not declare john, but at the same time allowing value undefined for both isaac & john, which is not what I wanted.

My goal is that if I defined isaac, or john, the value should not be accepting undefined, but has to be of type { name: string, age: number}, wondering if that's possible?