Choosing the right bed mattress is largely a personal choice but here are some things to consider when selecting your ideal mattress. The following points can be applied to mattresses for people of all ages and sizes.

The right mattress should be firm enough to support the entire body in a fairly level plane (not saggy so the body bends down in the middle like a banana).

Secondly the top of the mattress should be padded enough to allow prominent parts of the body like shoulders and hips to depress in to the mattress.

Test this in a mattress shop:

Have the intended user lie on their back on the mattress.  Have someone else slide a flat hand under the small of their low back, checking for how snug it feels.  Next have the person turn onto their side then perform the same check under the side of their low back, feeling for the snugness of the fit.  Compare the snugness of these two positions on all mattresses you are considering.  The mattress that provides the snuggest or tightest fit is likely to have the most suitable padding on top and therefore provide the best support.