Delta is very tricky. They will guarantee you a bassinet (meaning, they'll bring one on the plane for you) but won't put you in the right seats.
From my experience, only elal and united will actually confirm the bassinet and put you in the correct seats.
LY rule for infants is 9 months or less will get a bassinet (if still in quota). Otherwise you won't get it.
UA will give for older than that as well. (Not sure till what age)