When to start Potty training my 18 month old?? IS the signs there?

My son is 18 months. I don't know if its time yet or not. We have potty training seat in our bathroom. He'll sit on it and he'll play with the flushing it. Also he likes to flush our toliet. BUt no real sign of him sitting down when he's peeing or pooing. I know some kids train as late as 3 or so I hear. I just don't wnat the signs to be there and I"m over looking them.