I've got an obsession over low level legendaries so I compiled a list of the lowest level you could a own a legendary at

>Bird trio
Level 3, LGPE

>Mewtwo
Level 60, USUM

>Mew
Level 5, event. Level 7, RBY.

>Beast trio
Level 30, event

>Lugia and Ho-Oh
Level 5, Dream Radar

>Celebi
Level 10, event

>Regi trio
Level 30, Platinum

>Eon duo
Level 30, ORAS

>Groudon and Kyogre
Level 45, RS/ORAS

>Rayquaza
Level 50, HGSS

>Jirachi
Level 5, event

>Deoxys
Level 30, FRLG/Emerald

>Lake trio
Level 50, DPP

>Creation trio
Level 1, HGSS

>Cresselia
Level 50, DPP

>Heatran
Level 50, Platinum

>Regigigas
Level 1, Platinum

>Manaphy and Phione
Level 1, event and breeding

>Darkrai
Level 40, DP

>Shaymin
Level 15, event

>Arceus
Level 80, DPP