Michael started singing the day he was born, literally. He was born with an amazing gift to sing and dance. At the age of 5 him and his brothers became "The Jackson 5." I don't think there really was a time when Michael became interested in singing, he was born with it.
people have started singing careers in all sorts of ages ,like Michael Jackson started when he was more or less 5 and some other people may start as 25 or 36. You can try and start as long as you're confident enough about yourself.