I was under the impression that this was because the left skews younger, and Gen X and millennials are more active and capable online.
From what I've seen (living in US, Germany, and Sweden) no matter where you go there is a difference in 'wiring.' [0]And according to some sources I've read there appears to be some science behind that as well.
[0]https://www.theatlantic.com/politics/archive/2013/10/can-you...
I think there is an element of the left wanting to "fix" the world which appeals to young people, and the right wanting to "protect" the world from bad changes, which appeals to older more cautious people.
https://news.gallup.com/poll/118684/military-veterans-ages-t...
59% of millennials lean or identify as Democrat compared to 35% Republican https://www.people-press.org/2018/03/01/1-generations-party-...
I don't know a single person in the military today that is over 30, and I live right by Nellis(Not exactly hard data, of course). Most people go into the military right after high school. I'd be surprised if the average age is actually that high, especially considering you can't even join the military over certain ages - though that depends on the branch and other factors. IIRC you cannot join the marines if you are >30
I suspect you probably are specifically looking at officers, which is an entirely different story.
[0]https://www.ncsl.org/blog/2017/11/10/veterans-by-the-numbers...