This is such a tricky area, and I think it might vary from child to child too - as one of the posts said, children all mature at different ages. (phdinparenting.com)
Children mature and develop individually, so when to start potty training varies from child to child. (babybjorn.com)
The degree and length of separation anxiety varies from child to child, but if the separation anxiety is prolonged, think about whether a change needs to be made. (care.com)