Soldiers were indeed young then. That was a consequense of a culture that did not see adolescence as a given period of time.

On the contrary. The casting (and the performance) were perfect. In real life, military personnel are very young, on average, a fact that movies almost always hide behind middle-aged actors, in part, I'm convinced, to avoid informing the audience that warfare is horrific and not glorious or fun. And it's already been pointed out here that Matthew was more or less exactly the age of the man he portrayed in this film, so the whole situation is extremely authentic from that perspective.
Further, Shaw was the product of a comfortable, well-to-do Boston family, pumped into senior rank by his family's connections, wartime attrition, and political circumstance. He was literally a freshly ex-adolescent handed a giant scary job that his life to date had nowhere near prepared him to do, but that he was determined to do right. Matthew did an outstanding job portraying that quandary, from having to learn the difference between regulations and effective practice, to dealing with older officers who looked down on and ignored him.
The telling scene is when Undertaker, having just been made sergeant major, quietly tells Shaw, "Thank you sir, but I'm not sure I want it", and Shaw immediately responds, "I know just what you mean!"
Now that I'm old, I see myself at Shaw's age in every Broderick scene. The actor and director were both geniuses.

He was actually close to the historical age. But you have to admit he looked like a kid with a glued on mustache. I've known plenty of guys who looked much older and more mature than him at 18 let alone 26.

you realize Colonel Shaw was 25 at the time of his death at Fort Wagner. So I call it accurate.

That was historically accurate and part of the theme of the movie. As the Sergeant Major said he was just a boy.

We had 13 year old ship captains during the Civil War.

I dont dispute the age similarities, its just the real Shaw although young looking still had a "tougher" look.. Broderick is just too wimpy looking.

The real Col. Shaw also was young and they do resemble each other
https://blogs.harvard.edu/houghtonmodern/2011/08/25/if-i-should-fall-remember-what-you-see-here-robert-gould-shaws-letters-digitized/comment-page-1/
Sometimes a movie or tv show plot is so stupid that only the stupid can understand it.

To clarify things for you, Matthew Broderick was 26 years old during filming. Colonel Robert Gould Shaw was 25 years old when he died at Fort Wagner. He wasn't only young looking, he was actually younger than the actor.
The best diplomat I know is a fully charged phaser bank.

It was noted at the time that Broderick was too young but it was pointed out that the character he played was also young looking.
It's that man again!!

You would have really enjoyed being in combat with new 2nd lieutenants.
Perhaps the OP just wants to reach out for some sense of community.

In the intro of the movie it says he was 23, imo he looks about that age.

As another poster stated, Matthew Broderick was the appropriate age for this part.
Despite so many Civil War films and documentaries portraying the war being fought by middle-aged men (due to an over-reliance on the volunteer services of re-enactors), Civil War armies consisted mostly of very young men- teens and early 20's.
The war saw a number of very young men promoted to high rank. By war's end, the Union had a number of 20-something year old generals including a 20 yr old brigadier general (Galusha Pennypacker) and a 25 yr old major general (George A. Custer.) The Confederacy, likewise, had its share of "boy" generals. Thus, a 20-something year old colonel, like 25 yr old Robert Gould Shaw, was not exceptionally unusual for the Civil War.

ah, thanks for the info, i didn't know. that makes it more believable. will have to watch again with that in mind!

You aren't the first person to think this, but the portrayal is accurate because Robert Gould Shaw did indeed look like a baby faced kid in real life. And Broderick does resemble him. Shaw was in his early 20's same as Broderick.
I think the main reason people had a hard time taking Broderick seriously in this roll was because they still thought of him as Ferris Bueller, which came out only three years or so before this. Lets face it, its a huge contrast to go from Ferris Bueller to a Union colonel in the Civil War in three years no matter how good of an actor you are.