Edit: to clarify, Medicare does not only cover seniors. Other important groups include those with end-stage renal disease and those with significant disabilities. Those often also receive Medicaid, and are known in industry jargon as “dual-eligible.” Medicare becomes their primary payor, with Medicaid picking up the balance of what Medicare won’t cover. Both programs are ultimately overseen by CMS (center for Medicare and Medicaid services). Pay negotiations for Medicaid occur on a state level, but a huge chunk of the tab is picked up federally, so they tend to weigh in.
Newborns with SMA will almost certainly be initially covered by parental private insurance (pre-dx), and transition to dual coverage afterwards. Please note that there are a combination of gene therapies under discussion, only one of which will be given once at birth- the rest will entail ongoing costs that will be borne by Medicare and Medicaid.