its more a weight/size range - up from 9kg up to about 2/3 years(depending on size) they sit on an elevated seat(built in) with a 5 point harness and downward extended headrests, from then they sit on the elevated seat with the normal 3 point seat belt with extended headrest. When they are tall enough the elevated seat drops (pull one strap - and the cushion unfolds to become a normal adult seat) and the extended head rest can be removed (pull out headrest remove extension - replace headrest). The beauty is that it takes seconds to convert from one to another. As for fitting - they fit just like any of the middle/rear seats - pull lever to fold flat, pull strap to tilt vertical and then squeeze two bar and lift (mind you they are solid and about 35kg to lug around).