Why are there only OLD candidates for the US (vice) Presidency?

Barack Obama is going to retire soon and he's 55 years old right now. You'd think that - if it wasn't for the US laws forbidding it - he could easily serve another term or two.

Because if you look at the candidates for the presidency and their running mates;

- Rep: Donald Trump (70) and Mike Pence (57)

- Dem: Hillary Clinton (68) and Tim Kaine (58)

- Lib: Gary Johnson (63) and Bill Weld (71)

- Green: Jill Stein (66) and Ajamu Baraka (62)

you can't help but notice that they are ALL older than Barack and they haven't even been elected yet (and I didn't even mention 75 year old Bernie Sanders).

I can't help feeling that a younger candidate - mens sana in corporo sano - would be a good idea; somebody in tune with modern technology who understands social media and realizes that tv is kinda dying (watching stuff online when you want, where you want instead).

So ... why are there only old candidates?