In mature romance, characters aren't blank slates. Their history shouldn't just be a footnote; it should drive their motivations.

Use dialogue that is clear. Mature lovers are more likely to say, "I don't have time for games," than to spend three chapters wondering why a text wasn't returned.

Two people who were wrong for each other at twenty, but right for each other at fifty. This allows for a "now or never" emotional urgency.