Crib mattresses should be firm to prevent the mattress from conforming to the baby’s face and posing a suffocation hazard. Young children are not as prone to these issues. However, there’s still a potential danger. While they don’t need as firm a mattress as babies, a firmer mattress should be used at least until the age of eight. After that, most children are large and mobile enough that mattress type shouldn’t pose a threat. Children don’t weigh as much as adults, which means they don’t sink as easily into the mattress. What feels soft to an adult may feel firm to a small child. A mattress that’s rated medium to medium-firm is generally firm enough for toddlers and preschoolers.