I think it depends. For babies to start, they need to have very good head/neck control, and must be able to sit in the high chair (buckled up) well without flopping over, must also show an interest in food, etc. Milk is still the main source of nutrition at this young age.
I started my second daughter at 5 months because she wasn’t putting on weight, was having a bottle strike (since 2 months old), and she had very good motor skills and was stable by then. She was also very interested in what we were eating. The milk intake definitely drops when solids start, regardless of how much they eat, so this is an important consideration.
I started my elder daughter at 6 months because she was drinking milk well, and her motor skills took a bit longer to be honed.
So I feel every child is different, and you need to try to do what’s best for each one.