Seriously. Boosters are for Big Kids! Posted on July 16, 2014July 9, 2018 by SuperCarSeatGeek Boosters are for big kids! They not only need the maturity to sit correctly 100% of the time (even if they fall asleep) but they need the skeletal maturity as well. And don’t forget! Kids typically need boosters until they’re at least 4’9″ AND are 10-12 years old!