I don't think the age-group it makes much difference except in terms of feasibility. You need to have a sufficient number of cases (malnourished) and a sufficient number of exposed (i.e. disabled) and you'd probably want to avoid zero cells in tables. Both malnutrition and significant disability are rare conditions. These considerations would favour a broader age-range. I would not extend the age-range much above ten years so as to avoid issues of sexual maturation.
You may want to go for a study that finds cases of disability by (e.g.) chain-referral sampling and use familial and neighbourhood controls. This a bit "backward" as selection would be on exposure (disability) rather than outcome (nutritional status) but that should not be a problem. It would, I think, be more cost-effective, easier to plan, and you would have some control over the eventual power of the study.
One concern that I have is that by "malnutrition" you probably mean below some threshold of an anthropometric index such as weight-for-height. The problem here is that some disabilities (e.g. curvatures of the spine) make measurement of height difficult. Height-for-age has the same problem. If you use MUAC then you would probably want to pick the favoured arm. Weight-for-age might be better but you'd need good age data. Severe disability (e.g. absence of limbs) might make this difficult. What are your thoughts on this?
Why have the two sets of controls? It seems to me that age / sex matched neighbourhood controls might be easier and you control for age, sex, and "neighbourhood" (usually a melange of class, tribe, family, livelihood zone, &c.).
I hope this is of some help.