Many lifejackets are slightly different from one another, so look at the safety card to be sure. You inflate the lifejacket outside the aircraft or as you exit, I've never seen any you inflate before putting them on. Some airlines have children lifejackets (which are smaller) but same principle applies. For infants, I've seen different types of pod type 'life-cots'. Some you need to inflate before putting the infant inside, which you would do upon exiting, others you can put the infant in and then inflate.