The answer to this varies on a case-by-case basis. It depends on when your child got their twin bed, and how comfortable their twin mattress still feels for them. If their feet are dangling over the edge of the bed, it’s time to move up a mattress size. If your child has been sleeping on their twin mattress for almost a decade, it’s safe to say it’s time for a replacement. You’ll know when it’s time to upgrade your child’s bed, as they’ll either outgrow it or you’ll notice the bed begin to wear and sag with age.