To clarify, a man or woman (A) can never marry a woman or man (B), regardless of his or her age, if A had been married to that B's natural parent (C) and B lived with A and C as a family unit whilst B was under 18 years of age.
But there is no problem with them having consensual sex as adults.
See Table 30 here, with the sentence above it which states:
The remaining step-relatives can now marry provided they are over 21. Also, the younger person must not have been treated as a child of the older person's family and never, under the age of 18, lived under the same roof as the older person.
http://www.genetic-genealogy.co.uk/Toc11557014 5.html