How old are you supposed to be to become an artist?

One is never too old to become an artist. Grandma Moses was a renowned artist, and she didn't start her career until she was 76! The most important thing when it comes to art is not age, but passion.
It will take more time and resources to pursue an art career at the age of 40 as opposed to starting when one is younger, but it is still possible. Many people change careers two or three times nowadays, sometimes even four. Starting a different career at any age is hard and will take time, but that doesn't stop countless people from trying, and it shouldn't stop you. If your dream is to be an artist, then the best age to become an artist is whatever age you are right now. Don't wait another minute to follow your dreams.



FURTHER READING:
http://time.com/4482257/grandma-moses-history/

While being an artist can be seen as an “occupation,” it is most importantly a way of seeing the world, a visual (or auditory or verbal) sensitivity to the outside world, coupled with a desire to create something new, and a “work ethic” that prioritizes creativity over profit and fame. Art is “looking at things carefully,” as William Saroyan says, and an artist is someone who is sensitive to details more deeply than mere surface textures, shapes, colors, etc. (In music, the same things are true—rhythms, harmonies, and backgrounds; in verbal art, subtleties in sound, cadence, metaphor, figurative language). To your question, an artist is an “amateur” (literally "a lover of"), usually during youth and adolescence, often well into his/ her adult years. Of course, there are many exceptions (Mozart, for example), and young visual artists sometimes produce sellable work. The essence of your question speaks to the timetable for becoming a “successful” artist, one who devotes his/ her life to making something new, striking, and/ or memorable. In many cases, that concentration on one’s art results in some “commodity,” a sellable, remunerative “product” allowing the artist to make a living at it (Shakespeare, for all his artistic genius, was actually selling a commodity: live theatre). The age for this to happen can be any time.