To write a good 'age gap yaoi lemon fanfic', start with a strong plot that takes advantage of the age gap. Maybe the older character has a secret that the younger one helps uncover. For the yaoi aspect, build a believable relationship. Regarding the lemon part, it should enhance the relationship in the story, not just be there for shock value. Also, do your research on the yaoi genre and its tropes.
Writing a great 'age gap yaoi lemon fanfic' requires careful consideration. For the age gap, explore how it affects communication and understanding between the characters. In yaoi, create chemistry through their interactions. When writing the lemon part, keep in mind the overall tone of the story. If it's a more romantic - focused fic, the lemon should be a natural progression of their relationship. You also need to be aware of your audience and make sure your writing is appropriate for those who will read it.