I'm a sucker for Bucky Barnes and fics ab him  okay? But they can be really hard to get right. Trust me I PERSONALLY KNOW THIS. I've written like 5 and took them all down because they suck ass.

Your OC knew him back in the 40s and mourned his loss. And then somehow miraculously didn't age during the time between his disappearance and the avengers

Now we usually don't get an explanation for this just that your OC was madly I love with Bucky and is still heartbroken
After like 70 years

How is your OC not dead or super old?

We don't know

Because it's not relevant to the plot and you didn't think about it while you were writing an EPIC LOVE STORY.

I'm just saying that a little bit of background ...... goes a loooooooooonggg way