It has been just over two years since Bucky's return when he first starts to lose his balance.
It's a small thing, no more than a misstep during a training spar; but it's out of place for an assassin with 70 years of perfect kills under his belt. Steve notices, but says nothing. He knows Bucky will deny it and they'll get nowhere, so he watches and waits.
When Bucky actually trips on the stairs, he asks what's wrong, but Bucky waves him off.
"It's nothing. Just lost my footing for a second. Happens to the best of us."
It isn't until Bucky collapses and can't get up, during a run one hot humid morning, that he finally admits something is wrong.