Is it permissible to perform Istikhara concerning a one-sided love, and the resultant wish for marriage in the future, given a significant age difference between the two parties?

Love relationships and infatuation between young men and women are a gateway to temptation and a pretext for evil. Istikhara (seeking guidance from God) is not performed for involuntary inclinations of the heart, but rather for permissible, voluntary matters such as engagement and marriage. If the man has not proposed to you, there is no meaning in performing Istikhara, and you should not preoccupy yourself with him. Instead, turn your heart away from him and ask Allah to facilitate what is good for you. Occupy yourself with what benefits you in your religion and your worldly life.