Yes. Any age that they want. Most kids can start around 18-24 months, but you can try anytime if the child is willing. A good time to start trying is when the child shows readiness, i.e. Telling you they went or need to go, sneaking under a table to poop, etc. Start by putting them on at key moments like getting ready for a bath or when an adult is using their toilet.
Do Not Being Wet. I have found that when a child insists on a dry diaper immediately 80-90% of the time, they can be trained in 1-2 weeks.This can occur at any age over 15 mos . If they do not care about comfort, it becomes very difficult to train them.
Signs of readiness. are being able to walk from room to room, pull diaper, pull-ups & pants up & down, be dry for 2 hours, say "potty" or other word to indicate he senses the urge to go. Till then, let him sit on a potty, clothed, while you model using the toilet. Pushing it creates behavior problems. Buy a Little Looster stool. Read http://m.kidshealth.org/parent/emotions/behavior/toilet_teaching.html.
Yes. First sign is that the child needs to be a little uncomfortable in a wet/dirty diaper, so there's some incentive to stay dry. If he/she seems interested, i would make a big fuss every time there's a success to give a little positive reinforcement. I would also make it fun (reading on the potty, "target practice", eg). "what you're spending now in diapers, you're saving later in psychotherapy!".