These signs are the consensus across AAP, WHO, and major pediatric societies. The strongest predictors are physical: sitting up with control, head steady, tongue-thrust gone. Interest alone isn't enough — babies show interest in everything around 4 months but aren't physically ready.
Around 6 months is the typical right time. Slightly earlier (5–6 months) is okay if the physical signs are clearly there. Before 4 months is not recommended.
Sources: HealthyChildren.org (AAP) "Starting Solid Foods"; WHO "Complementary Feeding."