When did you switch to toddler mattress??

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I have the babyletto non toxic pure core crib mattress. It’s double sided. A VERY firm side for infants and a slightly less firm side for toddlers. My 10 month old sleeps on her tummy with her bum in the air and she is waking up with red marks in her knees. I’m thinking to flip the mattress. The firm side is like a rock and the toddler side isn’t much different to be honest. So I’m just wondering when everyone else switched mattresses.