What a trash chapter. Why would she leave him such a vague note? What is even the point? Did she just want him to be desperately trying to stare up her skirt all day?
@gigabarto
There is no such thing as a proper adult in a younger body. The body dictates the age. There isn't some older version of himself lurking inside like a body-snatching parasite or puppetting the body from above. He reverted age and kept his memories from the other future, but that doesn't mean any part of his body was carried back.
I'll put it this way: If he spent his time from age 20 to 30 learning how to play piano to a master level, he might still know the theory after rebooting the timeline, but he wouldn't be able to play the piano because his body never learnt it. It still has the reflexes, the muscle memories, the hormone balance, etc of the teenager he reverted to. If he had reverted to a point in his life where he had been very sad about something (like a dead aunt) that he couldn't even remember clearly in the future, he would still be upset after reverting, possibly without even knowing why.