Depends on where you're located. Different countries define disability differently. In the US, the Americans with Disabilities Act (ADA) is pretty comprehensive to include anything that limits major life activities. Some disabilities have precedent and so are officially recognized. But other things, such as more rare conditions, may not be officially recognized, but have a strong case for being included if discriminatory legal proceedings were to take place.

It's up to the individual to receive disability accommodation. If someone lives with an impairment that others would consider a disability, they're not obligated to do anything. If they're fine living the way they are and aren't concerned with potential discrimination, then they can just go on living as they want. Many people do.

Down syndrome, as far as I know, is officially recognized. So if someone has it, they should be able to seek out reasonable accommodations and be treated as a member of a legally protected class.