How to make the radio buttons work and output values in reactjs

You had some spelling mistakes, inconsistency with variables names and wrong logic at handleChange:

import React from 'react';

export default class App extends React.Component {
  state = {
    firstName: '',
    lastName: '',
    Age: '',
    gender: '',
    isSubmitted: false
  };

  handleChange = event => {
    const { name, value } = event.target;
    console.log(event.target);
    this.setState({ [name]: value });
  };

  handleSubmit = event => {
    this.setState({ isSubmitted: true });
    event.preventDefault();
  };

  render() {
    return (
      <main>
        <form onSubmit={this.handleSubmit}>
          <input
            type="text"
            name="firstName"
            value={this.state.firstName}
            placeholder="First Name"
            onChange={this.handleChange}
          />
          <br />
          <input
            name="lastName"
            value={this.state.lastName}
            placeholder="Last Name"
            onChange={this.handleChange}
          />
          <br />
          <input
            type="number"
            name="Age"
            value={this.state.Age}
            placeholder="Age"
            onChange={this.handleChange}
          />
          <br />
          <label>
            <input
              type="radio"
              name="gender"
              value="male"
              checked={this.state.gender === 'male'}
              onChange={this.handleChange}
            />{' '}
            Male
            <br />
          </label>
          <label>
            <input
              type="radio"
              name="gender"
              checked={this.state.gender === 'female'}
              value="female"
              onChange={this.handleChange}
            />{' '}
            Female <br /> <br />
          </label>
          <button>Submit</button>
        </form>
        <hr />
        <h2>Entered information:</h2>
        {this.state.isSubmitted === true ? (
          <div>
            {' '}
            <p>
              {' '}
              Your name: {this.state.firstName}
              {this.state.lastName}
            </p>
            <p> Your age: {this.state.Age} </p>
            <p> Your gender: {this.state.gender} </p>{' '}
          </div>
        ) : (
          <div> </div>
        )}
      </main>
    );
  }
}

Simple you have to update the value and name in radio button.

Check below code.

<label>
  <input
    type="radio"
    name="Gender"
    value="Male"
    checked={this.state.Gender == "Male"}
    onChange={this.handleChange}
  /> Male<br />
</label>

<label>
  <input
    type="radio"
    name="Gender"
    checked={this.state.Gender == "Female"}
    value="Female"
    onChange={this.handleChange}
  /> Female <br /> <br />
</label>

Hope this will work for you!