To get more reliable results with the age difference, use one of these in your prompts weighted to how significant you'd like the age difference to be:
(onee-shota:0.5) (onee-shota:1) (onee-shota:1.25) (onee-shota:1.5)
NOTE: (be sure to also add NSFW to your negative prompts if you want to avoid that part of the onee-shota tag)
