Hi chanpou,
Your daughter is at an age (preteen to early teen) where she can provide meaningful feedback about a mattress and a softer comfort layer would certainly be suitable if she prefers it. At her age I would tend to follow the same guidelines that I would for buying a mattress for an adult.
While there is no way to quantify how long any mattress will last for any particular person … if a mattress is well inside the comfort/support range that is suitable for a particular person and isn’t close to the edge of the range that is too soft when it is new (see post #2 here) and meets the minimum quality specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer.
It’s always more realistic to think of 10 years as a maximum reasonable expectation for any mattress no matter what the quality of the materials and then treat any additional time after that as “bonus time” because after about 10 years the limiting factor in the useful life of a mattress will often be the changing needs and preferences of the person sleeping on the mattress and even if a mattress is still in good condition after a decade … a mattress that was suitable for someone 10 years earlier may not be the best “match” any longer and may need to be replaced (or fine tuned with a topper in some cases). With higher quality materials throughout a mattress or for people whose needs and preferences or physical condition or body type hasn’t changed much over 10 years then “bonus time” or even “extended bonus time” with higher quality/density materials like latex is much more likely than with materials that are in a lower quality/density range.
Unfortunately there are too many unknowns, variables, and personal preferences involved to be able to predict with any certainty which combination of layers or components will be the best match for someone else in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either hers or a mattress) or “theory at a distance”.
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed conversation with a knowledgeable and experienced retailer or manufacturer who can help “talk you through” the specific options they have available based on the information you provide them, any local testing you have done (assuming you know the firmness of all the layers in the mattresses you have tested which may not always be possible to find out), and the “averages” of other customers that are similar to you (see mattress firmness/comfort levels in post #2 here). They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences than anyone else.
There is more about the different ways to choose a mattress that can help you identify and minimize the risks involved in each of them in post #2 here and there is also more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses in post #13 here.
If it’s a good match for her in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then it would certainly make a good quality/value choice IMO yes but I would make sure she has a chance to test it because she may find it to be on the firm side.
Again … a more detailed conversation with a knowledgeable and experienced online manufacturer will always be a much better source of guidance about which of the options they have available would have the best chance of success than I am. I would tend to think that most younger girls in her weight range would probably prefer the softer comfort layer but one teen’s specific needs and preferences may be different from the next.
Phoenix