I started at about 3 years old. I wanted my LO to be able to understand instructions and also take the initiative to verbalise when she needed to use the toilet. It worked well for me. Potty training at 5 months... I wouldn’t dare to attempt that. Haha. Your MIL is brave. Baby might not be able to even balance properly on the potty. And also cannot control the poo and pee or tell you when the potty is needed.
I started when my LO is 2.5yo
I don’t want to stress myself over potty training. My mother in law said she started her kids at 5 months!
Lavania Hajji