4 months is always what I was told. But it would depend on bub and when it seems like milk is no longer enough. When they don’t sleep long between feeds is usually a good indication

Most stuff I read said when bubs has good head control and can sit when supported, shows an interested in food. However, I started bubs at 4mths just to see whether he was ready. He didn’t have the greatest head control and since we never seemed to eat when he’s awake it was hard to know if he had an interest in food. I tried him with a spoonful and he gobbled it up. Now at 5mths we’ve just started three solids a day, he still doesn’t have the greatest head control but grabs my hand that’s holding the spoon and shoves it in his mouth.