The only reason that an adult would have a sexual interest in someone who hasn’t yet matured is because it doesn’t require as much self assurance to deal with as a more appropriately mature person. To a younger prospect it’s easier to appear more sophisticated, experienced, or worldly.
Well, if you’re saying that you are a fifteen year old (you don’t say male or female, but the motive would be the same either way) and twenty five year olds are hitting on you, you must appear vulnerable or naive and they’re having some fun at your expense. When I was fifteen, I had the same experiences quite a few times. I didn’t figure it out for years that they saw me as an easy target, easier to persuade and induce than someone their own age.