Well I'm not cabin crew but I've just consulted the Cabin Safety Manual for my airline.
All adult life jackets are suitable for adults and kids over 2. It also states that infant life jackets are recommended for infants between 0 and 16 kg. although we don't carry scales but I guess you have a pretty good idea about your child's weight.
I think Otto is talking about adjusting an adult life jacket for an infant, ie. 2 or younger.