Mag, there are advantages and disadvantages to giving semi-solids early/later. All depends on our bb's development.
From what I know, bbs with family history or personal history of allergies, should give milk longer. If bbs are given TBM, should also be given longer because of antibodies.
But all bbs should be given semi-solids at 6 months cos bbs lack iron. First 6 months, bb have iron from stores during pregnancy. AFter birth, bb's iron levels drop slowly. At 6 months, bb needs iron from other sources because milk is not high in iron.
Also, if we give semi-solids later, we need to go through the food types quicker than a mother who gave semi-solids earlier. Because bb needs to get nutrition from other sources instead of just cereal/porridge/safer types of root vegs.
Also, some bbs find that milk alone is not enough for them. In such a case, the baby will have lower than 50th percentile weight. The doctor will be able to advice in such a case. They keep on wanting more and more milk. So it's important to monitor overall feed over 24 hours to see if it is an increasing pattern. It isnormal for 6 month old bbs to have a growth spurt. But it is important to differtiate a growth spurt from milk being insufficient. You can tell the difference because growth spurts only last a few days, at most a week. if milk is insufficient because bb needs semi-solids, bb's weight will drop AND bb will continue demanding more and more milk beyond 1 week.
Having said that, it is recommended that bbs be given TBM for 6 months (unless doctor recommends otherwise) and up to 12 months (with semi solids), although studies show that bbs need at least 9 months of BM to derived optimum health benefits/mental development.
So it's a balance we need to strike.