There's about an eight-year difference between Steve Rogers and Bucky Barnes. Steve was 22 when he joined the war effort. Bucky Barnes was 14, even though officially on record it says he was sixteen. That's a lie.
But going into a war like that, you don't get to be a typical fourteen-year-old, you know? It's a war, there's stuff that no one should have to see or do during a war.
And... being an Avenger. You know, it's a bit like that too. You have to grow up fast. You make bigger decisions that impact a lot of people. You see people die. You save who you can.