Nikolai isn't necessarily 12 years old.
The phrasing leaves the meaning open to question: "He even saw his first battle at the border."
Not "He even saw his first battle" period. This means he could have seen battle before the battle at the border.
But this doesn't mean he was older than 6 when he saw his first battle at the border.
What does leave his age open for question is this line "that was about 6 years ago, wasn't it..."
This doesn't give us any information about the battle, not about when it started, not about how long it had been going on before Nikolai first got involved, and not about whether or not the war ended when Nikolai got involved, it could have gone on for many years after his first involvement, or it could have ended when he got involved.
In the latter case, he would be roughly 12 years old, but.
In the former case, the war could have lasted for many years after his initial involvement, which means we don't know how old he is, he could be 12, or he could be 20, he could be older than 20 or younger than 20.