The severity of symptoms experienced by XLHers varies greatly from patient to patient. Some are active into their old age, but others have mobility challenges beginning generally somewhere between early- and mid-adulthood.
Depending on the person's choice of career, those challenges can affect the ability to work. For those XLHers, it may be worth consulting an attorney who has experience with representing patients before the Social Security Administration (in the U.S., or the relevant governmental entity elsewhere). You're not likely to find one who has ever heard of XLH, but look for someone who has experience with presenting fairly complicated medical cases, possibly including other rare diseases.
Alternatively, the Social Security website has a LOT of information, much of it intended to be used and understood by non-lawyers. The main page for disability benefits is here: http://www.socialsecurity.gov/pgm/disability.htm
There's also a comprehensive set of Frequently Asked Questions and answers: https://faq.ssa.gov/link/portal/34011/34019/ArticleFolder/273/Disability
The Social Security Administration also has a resource called the Blue Book, which lists conditions it may consider in determining disability, and which can be found at their website The explanation is, in essence, that if an applicant has one of these conditions and isn't working, then it's pretty much assumed that the person is disabled.
XLH isn't one of the specific conditions that leads to a semi-automatic finding of disability, but that just means that the applicant needs to provide more proof of disability. Here's how they explain it:
[T]he presence of an impairment that meets the criteria in the Listing of Impairments (or that is of equal severity) is usually sufficient to establish that an individual who is not working is disabled. However, the absence of a listing-level impairment does not mean the individual is not disabled. Rather, it merely requires the adjudicator to move on to the next step of the process and apply other rules in order to resolve the issue of disability.The Blue Book is broken into sections based on biological systems, like endocrine and bones. Since XLH can affect the whole body, it may be difficult to pinpoint symptoms to a specific section of the Blue Book. For instance, there's a mention of parathyroid abnormalities (in the endocrine section of the Blue Book), but it doesn't list anything about low phosphorus, just osteoporosis, cataracts, kidney problems and excitable nerves. http://www.ssa.gov/disability/professionals/bluebook/9.00-Endocrine-Adult.htm
A lot of XLH issues fall under the musculoskeletal section, but there's no specific mention of osteomalacia (the adult version of rickets): http://www.ssa.gov/disability/professionals/bluebook/9.00-Endocrine-Adult.htm
There's also a section on "congenital disorders that affect multiple body systems," which would seem to apply to XLH, but it's not mentioned by name there.
NOTE: This posting is not intended as legal advice. You should consult with a qualified legal professional in your jurisdiction if you have questions about filing for disability.