I read that newborns should have very firm mattresses, while a toddler's mattress can be softer. I don't exactly know when a baby becomes a toddler, but when can they sleep on a softer mattress?
For example, if I get the Colgate Classica I, do I need to buy a different, softer mattress for when the baby is a toddler? I am so confused.

Re: s/o mattresses; question..
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I agree with L&R - toddlers don't NEED a softer mattress, it's more that they can safely sleep on one, whereas babies who are still in the age range for SIDS absolutely have to be on a very firm mattress (like the Colgate Classica - which, BTW, we have and really like).
For example, DS is almost 2 and is still sleeping on his crib mattress, but his "big boy bed" (which is a twin size, so he'll have it all through childhood) will have a normal mattress (not super-firm like the one in his crib) - just because we didn't think he'd want something THAT firm, long-term. However, if we were just moving him to a toddler bed, we'd probably just have him keep using his crib mattress, since that's what he's used to.
Make sense?
This is what I think. We'll be using the crib mattress in BabyBean's toddler bed and then getting him another mattress when he is ready for a big boy bed.
If you are concerned about the firmness of the Classica I consider the Classica II it's slightly softer and less expensive on babycatalog.com right now. I just saw the price drop and am considering ordering it tonight.
I don't really know about the Classica I, but I registered for the Monarch and its double sided. One side is much firmer for the infant and then if u flip the mattress the other side offers extra layer of foam padding for the soft growing bones of a toddler.
When we bought a crib & mattress for my SD, she was 1 yr and I remember being able to buy the less expensive mattress since she was older, so maybe the cutoff is around 1 yr. One year is also when the risk of SIDS drastically drops (although I think SIDS technically can go to 3, or maybe Im making that up)
Also, there are some companies now that make double sided crib mattresses, with an extra firm side for infants and a less firm side for toddlers. Moonlight Slumber makes all foam mattresses that are double sided, and they are the runner up to Colgate in Baby Bargains. We just purchased their cheapest option (Little Dreamer) last weekend.
And as PPs said, for infants it is a matter of safety, they need the extra firm mattress to help prevents SIDS. For toddlers, it is a matter of comfort, so you can definitely just use the one (extra firm) mattress for both age ranges.
We haven't - but maybe it depends upon whether (and for how long) you plan to use a toddler bed. I heard that they're kind of a waste of money, so we're going straight to a twin, but maybe if your DC is in a toddler bed until age 3 or 4 he/she might start wanting a softer mattress? At least so far, DS doesn't know any different, so he doesn't care...
That is a good question... we bought a double sided one as well, and I have no clue when would be a good time to flip it to the softer side! Although I guess that's something we probably don't have to worry about for quite some time, hehe.