Is it a condition for the person performing Istikhara prayer to be an adult, and are there any negative consequences after Istikhara?

Istikharah is seeking what is best from Allah and entrusting the choice to Him. The Istikharah prayer is valid for both the adult and the discerning child (the age of discernment is typically seven years), because acts of worship are valid from a discerning child. A Muslim should combine Istikharah with consulting people of religion and wisdom, and then rely on Allah and make a firm decision. Istikharah is entirely good for the servant and cannot have negative consequences.