I don't think it's a big deal, but I've always thought it was strange to call a man "kid" when they're over 18. Even 16 year olds are "young men."
Over the years, I've heard people refer to 25 year old football players as "kid" because "I'm older than him."
Of course it's subjective, but it wouldn't be common courtesy to call a grown man "kid," just like it's not polite to call elders "old man." (If it's not your dad, then don't say that...it's not endearing, it's rude.) I've seen strangers call elders "old man."
I get your point, a young man or young women can join the service at a very young age and give their life for their country, and I would never refer to them as a kid.
I think the only reason someone would be comfortable referring to a young adult as a kid is if they are talking about how young a man can be in the NFL and be done with his career. In the NFL a young man that is 30 years old is hitting his ceiling, and unless he is an acceptation to the rule, he is done in the NFL, but he is hopefully just a "kid" in life and has a long time to go before he is considered "old".
Not sure I would refer to a NFL player as a kid, but it really has nothing to do with his size, or his accomplishments, but more about what stage of his life he is in, and most NFL players are young when they "retire".
Another way of looking at it is if someone one passes away at 25 years old, I have heard it said " he was just a kid, he had his whole life in front of him" and I don't think it was intended to be disrespectful.