Is eternal youth worth having? A tech millionaire is taking blood from his own son to try and be young again. But what is so bad about growing old?
The man spending millions to be 18 again
Price of youth: Johnson spends millions every year on experimental treatments to de-age his organs. Glossary
Organs - Parts of the body that do a specific job, such as the heart or lungs.
AI - A computer programme that has been designed to think.
Disease - An illness or sickness.
Dementia - A syndrome associated with memory loss and other declining brain functions.
Eternal - Lasting forever.