Normally action and adventure movies are about young men (and sometimes women too) rushing around, pulling off feats of strength and timing to save the day.
If the actor is a little past his (or her) prime, then they act as if they're about twenty, and hope that nobody notices their laughter lines.
Robert Redford didn't bother with any of that. He played his role as a 76 year old man (though we're never told how old his character actually is). Nor was this portrayal lined through with zimmer frames and dementia. It wasn't there to mock the elderly.
All is Lost shows us precisely what older men are capable of doing. It's chock full of action, but occasionally the obvious solution may be thwarted by the fact that Redford hasn't the strength that he might once have enjoyed. The peril is heightened by the need - after pulling off a desperate but necessary feat for survival - to have a little rest to catch his breath.
Redford's character is very fit for his age. He is not at all in his dotage, but neither does he attempt to act like a teenager either. When his body fails him, the boatman diverts his energies into cerebral solutions, mining his experience or using tools to achieve his ends.
He's so good at it that the pace never falters and you forget that no-one's spoken throughout the movie. It's a clever performance riding a tidal wave of adventure, as everything that could go wrong does go wrong, until all is indeed lost.