What is a way that i can reserve a hotel w.o them knowing my age?

you the four of you all go. check in together. your cousin and your boyfriend can say one room is for them so theres one thats at least 19 for that room and you and your cousins girlfriend can say one room is for you, that way there is at least one 19+ year old "checking in". once you get into the hotel each couple can grab one of the rooms. they wont know and you can pay each other back.