No one, absolutely not one single person, from Mycroft to Anderson, believes that the kid is a de-aged Sherlock (except possibly character of your choice who saw it happen, but even then no one believes their story and assumes hallucinogenic drugs were involved). Because of course de-aging is impossible.
Sherlock's new age, whether he remembers being an adult, who the other characters thing this kid is, and what they think happened to the actual Sherlock is all up to filler. Make it as serious or as angsty or as funny or as ridiculous as you care to make it.
Gen or Any/any, de-aging of Sherlock
No one, absolutely not one single person, from Mycroft to Anderson, believes that the kid is a de-aged Sherlock (except possibly character of your choice who saw it happen, but even then no one believes their story and assumes hallucinogenic drugs were involved). Because of course de-aging is impossible.
Sherlock's new age, whether he remembers being an adult, who the other characters thing this kid is, and what they think happened to the actual Sherlock is all up to filler. Make it as serious or as angsty or as funny or as ridiculous as you care to make it.