Hi dhanley,
The quality/durability guidelines don’t mention anything about the total thickness of a mattress (only about the thickness of individual layers which can be part of identifying any weak links in the mattress) because the thickness of a mattress isn’t really a meaningful factor in terms of its durability or useful life.
The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person or child will depend on the specifics of the materials and components inside it. There is more about the effect of thickness in post #14 here but any mattress that is suitable in terms of firmness (medium or medium firm) and uses materials that are “durable enough” and “safe enough” would make a suitable choice for a child regardless of its thickness and all the rest would be preferences. Even a mattress that is only 4" - 6" thick that fit these criteria would make a suitable choice for a child and then you could add additional softness/thickness if and when they need it.
Some adults that have lighter body weights and that are stomach or back sleepers or prefer a firmer mattress could also do very well on some 6" mattresses in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) while other adults that may be in higher weight ranges or that are side sleepers may need some additional softness or may need or prefer a thicker mattress that is a better match for them in terms of PPP. As long as your mattress uses durable materials and hasn’t developed soft spots or impressions by the time they get older then it will still be in relatively good condition so you could add a softer topper if and when they need it as they develop into more adult proportions and weight ranges.
You may be getting overly involved with specs or “theory” or more technical information which can often lead to information overload and “paralysis by analysis”. While too little basic information can lead to poor choices … too much information or a tendency to overemphasize “specs” or “theory” that has little meaning to you in “real life” can be just as likely to lead to poor choices as well. You need “just enough” information to identify the main criteria that are important to you and to recognize the retailers or manufacturers that already know what you would otherwise need to learn about their mattresses and have your (and your child’s) best interests at heart. The knowledge and experience of the retailer/manufacturers you are dealing with can be one of the most important parts of a successful purchase and avoiding “paralysis by analysis”.
What your final choice will really come down to is …
-
Making sure that the mattress you choose is a good match in terms of PPP (medium or medium firm).
-
Making sure there are no lower quality materials or weak links in a mattress you are considering so it will last into their teens with the possible addition of a softer topper if they need it when they start to mature.
. -
Comparing your finalists for “value” based on #1 and #2 along with all the other parts of your personal value equation that are most important to you including the use of natural materials, the price, the importance of any return or exchange options that may be available to you and the costs involved, and how suitable the mattress will be for the “other uses” that you plan to use it for (some co-sleeping or sitting on the edge for “story time” etc).
Phoenix