Jonathan Evans is a member of a race called Chameleon, they're an advanced humanoid race who through technology learned through necessity how to steal other people's identities. He's not sure of his age as his original planet had a different cycle to earth and after a few time travel adventures it's made it more difficult to judge the issue.
Chameleons are forced to do this as a grant explosion on their home planet caused them to lose their faces and identities. To prevent themselves from dying out they crated a process to assume the identities of others. They went to earth and established a tour company near at an airport to abduct young humans.
They would put an armband on themselves and an abducted human, and would miniaturise and place these captives in suspended animation to prevent removal of the armband as the removal of the armband from the human would cause the chameleon who borrowed their form to be killed. The second doctor exposed them and offered to help find another way for this poor race.
Jonathan was one of the chameleons who only reluctantly joined in this, but didn't agree with a majority of his race who believed that they were the "most intelligent race in the universe" and that humans were only "as intelligent as the animals on their home world", he and a few others helped the doctor find another way. They ended up creating a device that would instead be worn as a pendant or pocket watch that could store a few compatible dna sources and would blend them to set their appearance.
Jonathan quickly settled on a form and became a pop musician in London named John Jones. He later followed the Eleventh doctor into his TARDIS because he wanted to confront him and his current companion about some negative thoughts they had on his performance. He joined in on a few adventures, and would often change his appearance. Eventually he decided he needed a break from his adventures and he wanted to return to his music career.
He ended up settling about 50 years after his initial entry to the TARDIS and changed his name to Jonathan Evans so he could start his musician career all over again, with some more modern music. Though recently he has found himself missing the adventures and has started to investigate near his gigs to look for any suspicious activities.