Life-stage difference as plot engine
Age-gap romance generates its best conflict not from the age difference in the abstract but from the concrete divergence in where each partner is in their life. One character is making choices that will define their next decade; the other made those choices years ago and is living with their consequences. This asymmetry creates specific plot pressure: the younger character's choices may require the older one to change plans they considered settled, and the older character's established life may require the younger one to skip stages they were counting on. Design your plot around the specific life-stage collision that is most relevant to these characters, and let the romance test whether the collision can be negotiated rather than avoided.