Ty had an interesting idea that he thought of because of his foster mom being kind of a jerk. His life kinda really sucked, and he didn't even know his birth mom, so... why not regress himself to before being conceived? That would really test the limits of how far he could change things.
"I wish I would deage and not stop until I get to just before my conception," Ty said. And he saw everything around him, and his body, shift around. His room changed to be all kinds of different configurations he remembered, going backward and getting further and further into the past. And in conjunction with it he shrank and shrank, regressing past puberty to be a child, and then steadily out of childhood until he was just a preschooler, and then a baby.
After a little while, he found himself naked and screaming on a cold birthing bed, and then was connected up to his birth mother and she pulled him up inside her moist, warm womb. It felt more comfortable than anything Ty could remember, and somehow he seemed to see her life go by in reverse as his development reversed. Somehow even though physically his brain was unmaking itself, his intellect was also totally unaffected, probably thanks to the magic of his wishing power. His fetal self grew less and less developed, its appendages weaning away, its genitalia and hair and head gradually vanishing until it couldn't be recognised even as a fetus.
Soon the embryo regressed to merely a fertilized egg, and at last the egg moved out of the womb and the sperm that had fertilized it withdrew. Ty wished for his regression to stop there. Now he could change his life and the person he'd grow to be any way he wanted...