The Final Frontier

The Final Frontier is referring to the last two doctors of The Multiverse: The 59th Doctor and The Last Doctor (aka The Frontier Doctors.) This picture here is 59 in his outfit ====>

The Frontier Series is mainly focusing on how The Multiverse comes to an end for The Frontier Doctors and why it happens. After the TARDIS was taken away from Theta Sigma for about 1000 years it wasted a lot of The Doctor's energy enabling him to only have 60 lives. (Canon.)

A few months after The 23rd Doctor regenerated into The 24th Doctor in The End of The Doctor, 24 meets a peculiar man in a mask who seems very suspicious. The Man in the Mask started to really mess things up. 24 couldnt let this happen because he knew that The Man in the Mask was a timelord. The Man in the Mask was turning The Doctors against each other and was creating chaos throughout The Multiverse. This decreased an even larger amount of regeneration energy from The Doctor seeing as The Man in the Mask was trying to waste their time. You can watch The Last Doctor's death in The Invasion of the Galaxy Part Two where The Frontier Doctor uses the last of his regeneration energy to blow up all of The Daleks to save everyone. The Last Doctor's first appearance was in the episode, "On the moon", but the appearance wasn’t canon.

The Last Doctor is around 6000 years old but in the Novel Spin-Off "Doctor Who: Spirit Reality", The Doctor (aka Theta Sigma) is just a teen because he was effected by the mass destruction of the universe. (Turning it into the Multiverse.) It really effected the way that The Doctor lived throughout his life, especially after The 45th Doctor regenerated into The 46th Doctor, but that is confidential until Christmas Day.