Here is a link from the U.S.C.G.
http://www.uscgboating.org/safety/life_ ... acket.aspx And
http://www.uscgboating.org/fedreqs/default.html From Assuie:
"Children under the age of 10 must wear a specified PFD at all times on any vessel when the vessel is under way and they are in an open area of the vessel. When choosing a PFD for a child, care must be taken to ensure that the garment fits the child and that small children do not slip out when placed in the water."
The above is true for the U.S.A. but if they are "below deck" and under 13 of age, not 10.
Also, it basicly it states, that there NEEDS to be enough like jacks for all on board!! AND, thet MUST be the right type (off-shore, coastal...) and size for that person. So is you have a 300lb person on board, you MUST have a PFD for that person. Same holds true for a baby!!
Personaly, I will not have any one under 5yrs old and older persons that are not stable on there feet. I am a little over 50 now, my friends are asking if the parents can come out with us or even there childen. Why, if any thing were to happen, being that it is my boat, I am responcable for that person!! I do not want that over my head!! PLUS the LIABITY.
As to the above post, "4 months old". If the boat was to go down, as the "capt" you are trying to save everyone, your self and the boat. At the same time the mother is in a panic. No thanks, not for me.