Kieran Seery is wanted in connection with the assault, which allegedly took place at a home in Accrington yesterday morning (December 25).
Police said Seery later attended a home in Dowry Street at around 8.30am, but has not been seen since.
Detectives believe Seery has suffered a serious injury to his neck that morning, which requires urgent medical attention.
Lancashire Police are warning anybody who sees him, or knows where he may be, not to approach him.
If anyone sees Seery, they are urged to contact police immediately.
Insp Andy Willis, of Lancashire Police, said: "We are asking anyone who knows where Seery may be to get in touch with us as soon as possible.
"He is wanted in relation to a serious sexual assault on a teenage boy which occurred on Christmas Day morning (December 25) at an address in Accrington.
"Please do not approach him, but contact us on 101 with whatever information you have.
"We would also urge Kieran himself, if he sees this appeal, to do the right thing and get in touch."
Anybody with information should contact us on 101, quoting log number 0291 of December 25.