I started my DD at four months because she was on formula and having major constipation issues that weren't resolved by trying different formulas. She only had pureed fruit though for the first couple of months and this solved her butt problem.

I didn't start DS until six months and even then it was a very slow start. He was probably 8 months before he was really interested and eating regularly. He was breastfed and didn't have much interest in anything else until that point.