The answer is, not before 17 weeks of age (4months) and not later than 26 weeks (6-7 months). That is what I recommend, starting at 4 months, specially for children with gastroesophageal Reflux Disease. http://ramnikm.sg-host.com/2014/03/29/reflux/
New Recommendations from ESPGHAN (European Society for Paediatric Gastroenterology, Hepatology and Nutrition)
https://www.ncbi.nlm.nih.gov/pubmed/18162844
WHO says 6 months (May be updated at some point)
http://www.who.int/nutrition/topics/complementary_feeding/en/
UNICEF has a nice leaflet
https://353ld710iigr2n4po7k4kgvv-wpengine.netdna-ssl.com/babyfriendly/wp-content/uploads/sites/2/2008/02/introducing-solid-foods.pdf
