Este assunto é tratado neste link em inglês, do departamento de patrulha rodoviária da florida.
Pelo que li e entendi, texto abaixo, até 3 anos, utilização de assento para criança, de 4 a 5 anos pode usuar o booster.
. For children aged through 3 years, such restraint device must be a separate carrier or a vehicle manufacturer’s integrated child seat.
2. For children aged 4 through 5 years, a separate carrier, an integrated child seat, or a child booster seat may be used. However, the requirement to use a child restraint device under this subparagraph does not apply when a safety belt is used as required in s. 316.614(4)(a) and the child:
a. Is being transported gratuitously by an operator who is not a member of the child’s immediate family;
b. Is being transported in a medical emergency situation involving the child; or
c. Has a medical condition that necessitates an exception as evidenced by appropriate documentation from a health care professional.
(b) The department shall provide notice of the requirement for child restraint devices, which notice shall accompany the delivery of each motor vehicle license tag.