I think we are meant to presume they walked up the mountain
I'm not sure it actually matters if they do or not - I think the desire to do so and the sure belief that they still could (however many good reasons they may keep finding for never actually doing so!) is all that counts.
There's the undercurrent that they're bidding us all farewell - or should that be cheery-bye?
- before they've reached the top of the mountain. i.e. they may be old and no longer in their physical prime, but they haven't given up and they still soldier on.
They've let us in on so many great moments of their lives, but they don't exist just for our pleasure and we go our separate ways with them still having further adventures ahead of them, albeit we obviously know that they won't be around forever; and yet, by graciously taking a bow and parting now, in a way, they WILL still be around forever. Still Game, in fact 
I thought it was very poignant at the campfire when it sounds like the weather will be too bad to climb the mountain the following day:
Boabby: "Guys, Isa's right - I don't think any of us is going up there the morrer. Looks like it's gonnae turn."
Jack: "Well, that's that then, eh?"
Victor: "Och, there's always next year."
Jack: "Aye, next year.... that's if any of us are still here next year."
Victor: "Of course we'll be here next year; none of us are going anywhere - just like that mountain."
And then, right at the very end of J & V's last scene:
Victor: "Do you think they'll be wondering where we went?"
Jack: "Nooo. They'll know where we went."