What is the difference between Judo, karate and martial arts? Is there any age limit to learn judo or martial arts?
"Martial arts" is a name for practices that aim at perfecting fighting skills. Some people include things like boxing and judo, while others would say they are "martial sports" as their practice focuses almost exclusively on techniques allowed by their competition rules, clearly ignoring considerable dangers from other techniques.
Judo is an art of throwing, tripping and wrestling. You should search youtube for some judo videos. For a martial art, judo is fairly uniform in the way it is practiced internationally, as the Olympic sports rules provide a shared focus.
Karate is very different from judo, as it emphasises striking techniques such as punches and kicks, elbows and knees, and corresponding blocks with relatively few throws or wrestling moves. Again youtube can give you a taste for this. There are many, many forms of karate that are very different in emphasis and quality, as there is no international body coordinating standards. Some styles have competitions - sometimes open to other styles and sometimes not, other schools are only interested in (relatively) cooperative free or set sparring. There are no unifying rules across all karate styles.
Each school has to decide for themselves whom they will teach. You do read about people who apparently "started training" at age four or five under their parents, uncles etc.. In my first school we wouldn't accept students under around 10. My current school has students who are very young - 5 or 6 perhaps. It's quite shocking to watch them doing full contact fighting, albeit with lots of padding. I think somewhere around 12 to 16 is a good age to start training... starting earlier doesn't necessarily lead to better skills later, and being of an age where you can make more progress without being so limited by your body, attention span, knowledge of logic, science/mechanics etc. can avoid boredom and stagnation.
You have a reputation of 1. This seams like a very noobie question.
Martial arts is the category of all, um, martial arts. There are more martial arts than I could ever know.
Karate and judo are both martial arts. I'll name a few as well:
Karate, taekwondo, ju jujitsu, BJJ, Kung fu, aikido...
Of course, there are many different types of specific martial arts. For example, there are 8+ different types of taekwondo. Kung fu is even more (examples: wing chun, Jeet kun do (JKD))
Excuse my spelling, apple's autocorrect is only so good.
There's lots of different martial arts from each country as well. For example: Korea. Here's some Korean martial arts:
Tang soo do
There are even martial arts that aren't very normal. Take caprera (?). It's disguised as dancing, but is actually a dangerous martial art.
Not all martial arts are Asian. There even Israeli martial arts, as well as the Brazilian version of JJ.
There is no age limit for martial arts. If you are *older, I recommend taking aikido. It's extremely fun and effective.
Karate is considered by many martial artist to be an American martial art. It is more popular here than in in Japan. Most American karate schools have little to no ties with Japan, and little respect for the ancient art. American karate has become (generalization) a scam. A ripoff. A money making BUSINESS. The McDojo.