You need to earn age points. to do that you need to either play games or complete questes.

on one of the missons u will find ho-oh. U wll figure it out after u have beaten rayquazza and did most of the questes like beat lugia,kyogre,entei.raikou,suicune, and other legenderies. I want to know how to recreute him ( I WROTE THIS QUESTION) Just battle it at least twice, and after awhile, it will join! i got ho-oh on the second try. Some are harder, like rayquaza, so just keep battling them! (ho-oh, lugia, mewtwo and jirachi are the easiest to recruit) P.S. Try to take a key into ho-oh's dungeon, so you can get the friend bow on 30f. Equip it to your character, it makes all legendary's easier to recruit.
at the peak of mt. faraway (40th fl.)