Board Thread:Character Discussion/@comment-7526254-20160510162817/@comment-4260948-20171023230236

Emma says that she and Snow and David are the same age (3x01) when they're on the boat (and Emma is still 28 at that time) so by means of deduction, there were 18 years between Regina saving Snow (aged 10) and the curse being cast (age 28). 28 - 10 = 18.