Yes, a 40-year-old person can certainly buy a Royal Enfield bike. There are no specific age restrictions on purchasing a Royal Enfield or any other motorcycle, as long as the individual meets the legal requirements for obtaining a driver's license and abides by the relevant regulations for operating a motorcycle in their jurisdiction. As long as the person has the necessary qualifications and fulfills the legal obligations, they can purchase and ride a Royal Enfield bike at the age of 40 or any other age.