It has to do with their digestive system which isn’t fully matured yet under the age of 6months.

All of my kids have started solids at 4 months, even my two premmie kids (advised by our paediatrician), if your child is looking ready for solids (purees) then give them a try, if they reject them try again another time, keep in mind they change the recommended start age all the time between 4 and 6 months. Do your research and use your best judgement

They cant digest solid foods and may not be able to eat them properly so purees are best until they are ready

babies under six months have trouble digesting solid food as their digestive system is not strong enough to handle food. it can cause a lot of upset tummies. Much better to wait until they are over six months.

Have a research into open gut and it may help you understand why 6 months is recommended. Starting early can cause alot of trouble further down the track with tummy issues.