There is no end age limit. I have done it in ages of 9-65 years.  It truly is a very powerful procedure that many people dont truly understand.  

I believe the results are better in adolescents and young adults before degenerative changes occur in the midtarsal joint. If this is the case one can expect pain complaints after calcaneal osteotomy alone, and I believe midtarsal joint arthrodesis should be considered. 

In my view, no age limit but prior to degenerative changes in midtarsal joints

Evans procedure was modified 20 years ago by Mosca and we can compare this osteotomy with Salter osteotomy due at the hip joint. After the lengthening of the external ray the coverage of the talus head increase by correcting the abduction of the forefoot.  To be efficient is important to have enough bone that means that exist age limit. I think 9 years old is ok to start this osteotomy if we treat a child with spastic disease. For idiopathic planus valgus is better to use this osteotomy after 12 years old at girls and 13 years old at boys.

Thank you, collegues !