"I'm (old or fat or out of shape or handicapped in some way). Is it worth starting karate (or some other martial art)?"
Now my strong suspicion is that this isn't a real question - the person knows that it's okay to start karate, no matter how old or fat they are, but they want some encouragement and positive feedback. And that's fine, I have no problem with people using social media to get some metaphorical massaging from strangers.
But the real answer is twofold:
1. Of course it is. Literally no matter who you are (maybe other than an actual quadriplegic), you can train in martial arts - and if you do, you'll soon be much, much better than you are now!
2. Probably not. No matter how hard you train, you'll never be as skilled as a truly gifted athlete who has been training since youth. You literally will never catch up to them and be as skilled as they are!
Which one is the real answer? Both. Because the question is ambiguous.
If you are only willing to start martial arts training if you have a reasonable chance of being one of the best in the world at it, then yes, you're probably too fat, too old, too out of shape, and too un-talented to start.
BUT if you want to study martial arts because you're interested in the myriad benefits of martial arts training (better physical health and fitness, improved self confidence, better mental health, improved capacity for self defense) then, unless you're a quadriplegic (sorry to quadriplegics), you can absolutely gain those things.
If you want to be better at martial arts than other martial artists who have been training since childhood, your chances are slim. If you want to be better than the general population, your chance is almost 100%.
The 'trick' to being happy with your training is recognizing what you want, and to whom you should compare yourself, and realizing what your expectations should be.
I will never be able to win a UFC match. I would absolutely win a MMA fight against 90% of my peers (meaning, men of my size in my age category) - because most of my peer have absolutely no training. Compared to my peers who train in martial arts I am probably below average (I've taken long breaks from training). Maybe I'm average? I'm not really sure.
BUT I guarantee that I'm much, much better at martial arts, and more fit, and healthier, than an alternate version of myself who didn't train at all.
So the real secret to success in martial arts is in carefully choosing how you evaluate your success! If you only think you're 'good' if you're holding a UFC belt (or have won an Olympic medal or an international tournament) then you'll most likely never be happy. If you are happy knowing you're better than you would have been without training, then you're guaranteed to benefit from training.
Osu.