The guy at 55 knows what he can give, he's a known quantity at all levels. He will likely hit the ground running without requiring extensive training, because he's seen it all before; and once he's settled, he will have no incentive to move on -- he stopped dreaming about jobs at Google/FB/Twitter a long time ago. If he has a problem, he'll just say it straight to your face with no anger, because he's tired of management bullshit and just wants to Get Shit Done. His kids won't need trips to the doctor/school/footie practice, because they're old enough to own their own cars; and that means the family doesn't have to go on vacation on that same school-holiday week all other young parents are fighting over.

The guy at 25 could decide at any moment that he really wanted that travel-writer career after all. He could find out he's so awesome with Scalisp 2.3 that TrendyCo is ready to sign him at 10x his current salary; or that he hates Scalisp so much he'd rather volunteer at Ebola clinics than write one line more. He could get married, have babies and see his productivity killed by sleepless nights, marital arguments, a divorce. More importantly, he will likely get bored after a few months and start angling for org changes you don't really want to have. When he has a problem, he'll just sulk because he doesn't know how to bring it up, and maybe he'll spread some venom around so the team as a whole will also suffer. He'll want to replace battle-tested solutions because he doesn't understand them, likely hitting again all bugs and corner cases that made the original implementation hard to understand. And after a few years, he'll piss off to double his salary because he has 3 years of experience in Rubynodexpress.js so qualifies as Lead Ninjastar for ShinyCorp.

When you look at it that way, why would you not hire the 55 guy?

I always see this argument but find it possibly disingenuous. Given the rates of turnover for programmers, or even non-programmers, I don't see how people can realistically think this. Young programmers have even more incentive to change jobs more frequently.

It's easier to let the data speak for itself, from the BLS:

  the median tenure of workers ages 55 to 64 (10.4 years) was
  more than three times that of workers ages 25 to 34 years
  (3.0 years). A larger proportion of older workers than 
  younger workers had 10 years or more of tenure. Among 
  workers ages 60 to 64, 58 percent were employed for at 
  least 10 years with their current employer in January 2014,
  compared with only 12 percent of those ages 30 to 34.

My money would be on the older person sticking around longer. It's very uncommon for anybody to work at the same company for decades any more, but older people tend to have children, mortgages, and other commitments that keep them from jumping around too much.

Besides that, younger people will require larger raises to keep around because the difference between 1 year of experience and 3 years of experience on a resume is significant, while 30 years vs 32 years isn't. The older person will ask for a higher initial salary, though.

I'm also skeptical the 25 year old and the 55 year old will have the same skill level, assuming the 55 year old has been in the industry for any significant amount of time.

Edit: I didn't see "junior" in your comment when I wrote my reply.

I would go for 55 without second thought.

It would be for the ‘wisdom’ and life experiences; ability to take best decisions when no data is present (better intuition due to more life experiences); knowing that the person would be less likely to quit due to impulses; probability of having better intra personal skills; and so on…