You might want to take a look at what I wrote about the subject in my answer here:
Defence against Wing Chun
In the link above, I explained what the original purpose was for Wing Chun, why it was invented, and what its weaknesses are. I think you can apply that article as a partial answer to your question.
You also mentioned self-defense vs. MMA fighting. For that, I would direct your attention to another article I wrote here:
Are there any effective Kung Fu fighters in MMA?
In the article linked above, I talk about why kung-fu in general doesn't appear very often in MMA. Change "kung-fu" to "wing-chun", and it still applies.
I also expound on what I consider "realistic" self-defense training at the answer below. Just ignore the top part about Taekwondo and jump to the part where I say "Moving right along...":
is Jun Chong TKD a legitimate TKD dojo for self defense?
Okay, now having gotten that out of the way, I just want to say some things about the legitimacy of Wing Chun as a self-defense style.
You can train in any martial art and use it for self-defense, even a martial art that has just one punch, and that's all. That one punch could work if you do it just at the right time and the right way.
The problem is that you won't know what the right time is, unless you've spent a lot of time practicing it against partners that are trying relentlessly to do everything they can do to beat you. They get to do anything to you: punch you, kick you, grab you, lock your joints, throw you down, choke you, pin you to the ground, etc. And you just have your one punch.
If you train that way, you're going to do just fine, even with that one punch. Well, you're going to do a lot better training that way than if you just punched to the air or practiced on a compliant partner who lets you do stuff to him.
It's how you train that matters.
What MMA does is to remove the "model" about how a martial art thinks a fight "should" look like. It replaces dogma with something that has no model, just rules for what you can't do. Outside of those rules, you can do anything. And that's very useful, because it very closely approximates what you're going to deal with in an actual self-defense situation.
And by the way, the rules of MMA (or Gracie JJ) allow you to practice day after day without getting badly injured. That's important. It means you can go all-out in your training, and it's still more or less safe. If it wasn't safe, you would spar maybe once or twice a year. And then you'd head to the hospital afterwards. How good do you think you'll get sparring twice a year? How much better would you be if you could spar every single time you train?
Wing Chun can work for self-defense, like anything else can work. And it actually does do sparring on occasion (some places more than others), which is better than just practicing forms, chi-sao drills, wooden-man dummy, and so on.
The problem is how Wing Chun people train. If they train with MMA people using MMA rules, they're going to figure out very quickly what works and what doesn't. Unfortunately, most Wing Chun groups don't do that. They train using traditional methods.
As for performing Wing Chun as you get older, it's one of the least physically demanding styles out there. You're not going down into any splits. You're not doing high kicks. Your not doing back arches. Yes, you do lean back in most styles of Wing Chun, but that's going to be a muscular limitation, rather than a limitation on your flexibility. People who do T'ai Chi actually have to go through a much larger range of motion, in general, than Wing Chun typically requires. And an awful lot of elderly people do T'ai Chi.
Where I think older people will have problems with Wing Chun is in its hardness. If you have an instructor who demands you hit the wooden-man dummy hard, or he has you hitting each other hard during chi-sao or sparring, that's a problem. The older you get, the less able to do hard training you will become. It really depends on the person and the instructor.
My general advice is: Do what you think you're able to do. Your instructor may insist you go harder. Don't if you think it's going to hurt you. At age 50, for example, your joints and tendons will be a lot less forgiving than at age 20.
Hope that helps.