All players have a ceiling, so that is one issue. I'm exactly your age, and I am probably weaker and certainly no better then I was at 18, even though I have worked at it off and on over the years.
The other issue is that many people just play without making a concious effort to learn. You will pick up some stuff along the way, but there is a limit to how good you can get doing that.
I think the way to learn requiring the least dedication for a decent effect is to play at a chess club every week with players of different strengths, where its customary to have a "post-mortem" after the game. There are other good ways and specific areas to study (books, software, tactics problems, openings, endgames) but a chess club is what I found most effective (in getting to class A anyway, others who have gotten further can feel free to disagree.) You may also need to do some of these other things to get to class A, all I'm saying is that a chess club is good value for your time spent and fun too.
I generally look through my opponents' last few games just to get a sense of who they're playing and if they're hot or in a slump and whatnot. It's probably not useful info, but I just get curious. Anyhow, I've noticed that at my level there are quite a few players whose ratings have not improved over long periods of time. Even though they seem to be quite active and playing a lot.
I don't want that to happen to me. Is there some common mistake that people make in their chess lives that inhibits progress? My plan for improvement is to focus on the endgame and tactics while limiting myself to a pretty narrow opening repertoire. And of course, playing games. Is that a disaster waiting to happen? Could I spend my time better?