There is no age limit. Photography is an art and different people have different ideas and thoughts to view a thing. If you have good senses and you are creative then you can become a good photographer at any age. A child with good photography sense can be good than an adult who don't know what the the best view to make a photograph.

 There is no age requirement. Being a professional just means exactly that, your profession. Which means you make a living from photography. If you can get your stuff in print and get paid for it, then you are a professional.