Mine was 1

Usually when they start showing signs of interest: taking their diapers off, beginning to hold it and do a "potty dance", being curious when you're in the bathroom, etc. it's really different for each child. And some don't show any signs so ultimately I would say start whenever you feel is right. We tried at a year for our oldest because she started showing signs but the big toilet scared her and she wouldn't even use the toddler toilet lol. Then we tried again at almost 2 years and she finally got the hang of it. Then she was completely trained at night by 3. So she took a little longer than average despite showing signs earlier than the average.