I recommend when the baby is ready, usually between 4 to 6 months
The following resources support that:
https://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/expert-answers/starting-solids/faq-20057889
https://www.uptodate.com/contents/starting-solid-foods-during-infancy-beyond-the-basics
This AAP article has a nice video in it:
https://www.healthychildren.org/English/ages-stages/baby/feeding-nutrition/Pages/Switching-To-Solid-Foods.aspx
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3710027/