Like real people, characters get old. Or should. If a book only covers a couple years of a character’s lifespan, then that isn’t a problem. But if the author is young, and they have an old-ish character, they seem to make one of two errors:
Either their older character is afflicted with every old-age problem known to man, and a few we don’t know about, or the older character only talks about being old, and otherwise is as energetic and healthy as any young character. An author needs to have experience and in this case, most young authors don’t know what ‘getting older’ feels like.
I am no longer young, although I reject the idea that I am old. Still, I am beginning to experience some of those ailments that creep into the bodies of ‘old people’.
Arthritis is frequently thought of as an old person affliction. Mine started as occasional sharp pains in a knee, sometime during my 30s, but it really started slowing me down (literally, I could hardly walk more than 6 steps) in my mid-40s. That could have been earlier than most people, while a few people might not start experiencing it until their 60s or 70s. My knees are much better now (they’ve been replaced), but now I’m battling arthritic bone spurs in the shoulders that reduce my range of motion. And every time a thunderstorm is coming, every joint in my body aches - neck, shoulders, elbows, wrists, fingers, knees... I suspect my ankles and toes ache, also, but their message of pain can’t fight its way through the messages coming from so many closer-to-the-brain locations.
Also during my 40s, I started needing to use reading glasses. This included not only reading, but sewing, painting, computer work... The list is practically endless. I have slowly progressed to a stronger and stronger magnification. If your older character doesn’t have glasses, he/she might lean closer and/or squint to try to bring details into focus. On the other hand, my dad used to hold the newspaper at arm’s length and squint, trying to focus his eyes, and he died around 49. So your character might do it either way, but probably not both.
If your story takes place in the future, medicine might have cured these ailments, or delayed their arrival for decades, in which case, you don’t really have an older character. If your story is in the past, people died much younger, and these afflictions arrived much earlier.
Old people’s immune systems gets worn out, so they catch things easier and keep it longer. Gout is extremely painful and keeps that joint from bending for days. (These days, steroids send it packing quickly.) Do a little research on old age afflictions, and then pick a couple for your older character to deal with. Don’t load him/her down with every illness; it’s not only unlikely, they would be pretty useless. And don’t forget the common cold, flu, and all those other illnesses that even younger people have to contend with.
Make your character’s older age believable, but don’t make that character a burden to the rest of the war party. Unless, of course, that’s integral to the story.