Which of the martial arts is the oldest? And which is the youngest? And which is the most average age?
If you invented another martial art, how would you get it appraised, approved, accepted and deployed? Is there a naming convention, or can you make them up?
Who did the first kick, and who was it aimed at, and did it make contact?
For oldest, please refer to What is the oldest documented Eastern Martial Art that is still practiced? and What is the oldest martial art?. Also see What qualifies a school or business as a legitimate martial arts system?
In general, there is no formal certification or approval process for martial arts. There may be rank systems within an art like karate or judo, but these are not recognized outside their respective organizations. Anyone can basically open a school for martial arts and call it whatever they want, regardless of whether they have any knowledge or skills. In some areas, there may be a local group like a guild that decides who can open schools, but this would be region-specific.
The de facto standard for appraisal and acceptance is fighting, in a venue like mixed martial arts (MMA). If you can beat other people, then others are likely to want to learn from you.
Because there is no certification process, there is no way to quantify what the youngest martial art is. Somewhere in the world, someone has decided that the things they have learned now constitute a new, unique martial art and probably named it after himself. I would wager this happens a lot.
It might be hard to say and very hard to track but if we would speak in general and in a bit more relaxed terms I would say that Indian Kalaripayattu or in some versions "Kalari-payat" is one of the styles which was before majority of organized family traditions and styles as we know them today. Although western sources and some publications speaks around 500 BC or 2500 years old style there are Dravidian terms and words (within the languages itself) which are very similar to Kalaripayattu and which, when translated directly mean "somebody or one who learned to be very skillful in hand and legs-exchange improvisation" or in other cases "one who teaches others to survive anywhere without fear" and that terminology, according to some translators means they are speaking about specific type of "hand exchange and survival" and that art is much older (around 3000 years BC);
Some authors speaks about 28th monk after Buddha who went from India to China to teach Buddhism (Bodhidharma) and who meditated for 9 years in a small cave above Shaolin after which he gave monks first 7 fists of Shaolin (which is later discussed that those 7 fists must be Sanchin kata) I but it is known how there was already few hundred styles when he arrived in China so nobody really knows where the first spark of proper organized and develop style started.
It is knows who made certain upgrades and who contributed to the pool of knowledge (like yellow emperor) .. but real starting points are ancient really..
