Calculating according to the solar calendar

A boy who has not had a wet dream becomes baligh at the age of 15. I am aware that this refers to the 15 years in the lunar calender, but since people generally do not know their age in the lunar calendar, can we instead just tell them "15 years", even if they will assume this to be according solar calendar? This since it is difficult for the awwaam to calculate their age according to the lunar calender. Similarly can we tell them that "A child should be made to pray at the age of 7", "the custody of a girl moves away from her mother at the age of 9" etc, even though we know that they will assume this to be solar years? Also, can we ourselves act on the solar years too, even though we are aware that lunar years are really meant?