If I could change CA:TFA I would have made Bucky a teenager like he was in the original comics.

Personally, I really didn't like the change of Bucky and Steve being the same age. Their relationship didn't really hit for me and I think it would have been better if Bucky was younger.

And this wouldn't have been unprecedented.

In both WWI and WWII there were soldiers, even as young as 14, who had lied to join the military and serve their country. 

Imagine if the film had it that Bucky was around 16 years old and both he and Steve had been trying multiple times to get into the war, lying on their applications: Steve lying about his health and Bucky lying about his age.

Then Bucky gets in, and Steve watches as his friend who is like a little brother to him, is going off to war...a kid...while he's still at home collecting rubber or something.

Then when Steve goes to save Bucky, it's not just a friend saving a grown man, it's Steve rescuing a brave teenager.

And when Bucky "dies", have it be right after his 18th birthday, with Steve and the Howling Commandos lamenting that he had barely become a man.

This would also make it more dramatic for the audience when Steve encounters The Winter Soldier....as it's not just the Bucky we knew...it's Bucky grown up. Because the reveal of the Winter Soldier in the MCU was pretty hollow for me.

Now I would still want Sebastian Stan to be the adult Bucky in The Winter Soldier and the subsequent films. But I would have liked a younger actor to protray Bucky in The First Avenger.

I Wish that Bucky was a Teenager in Captain America: The First Avenger