When children turn three years old, this is a popular time for many to transition to a larger bed. There’s no rule about which size mattress you choose for your 3-year old, but if you plan on buying a mattress for them to “grow into,” you want a bigger mattress. The size bed you choose is dependent on your budget, the space in your child’s room, and how long you intend to use the mattress. Every 3-year old is different, so do what’s best for your child.