Handling a spouse, live-at-home children, and a single family home means that you are more likely to have given over much of your life over to a huge pile of responsibilities. I would guess the average forum poster is not a married,kids,home type. Single people are more likely to have the time to make psephology and/or racking up thousands of posts about politics on a message board a hobby. Heck, I bet some people leave this place after they get a significant other.
You're describing selection bias. That might explain some of it.
On the other hand, I've seen other online forums (for example a couple DIY forums I visit) where a lot of the posters are middle aged with houses, spouses, and kids. House/spouse/kids take up time, but not every waking moment.
So, selection bias for all forum-posting is probably too broad. I think it's probably a "seed culture" thing. Plus, as you get older you realize these ideological antler-rasslin' sessions are usually a waste of time, and what's an even bigger waste of time is saying inflammatory things online for the "naughty fun" of seeing people gasp and sputter. This means if you're going to talk politics online, you tend to either just seek out enclaves where people agree with you and can talk about "what to do" or else you seek out calm, polite, intellectual venues where you can quietly and slowly push around ideas. I don't know yet if this is one of those, still finding out.