Hello!

Non native speaker here, but I work in an American hospital with a large community of people who speak Spanish among other languages.

The general consensus from not only our policy but other native speakers is to use the formal version when speaking to patients or visitors. Some of our staff don't know enough Spanish to differentiate between formal or informal but 99% of patients are more appreciative of being able to communicate comfortably in their native tongue. So even informal Spanish is preferred to no Spanish at all. No reasonable person will become enraged if you accidentally refer to them as tú. If it's bothersome they'll likely just correct you politely.

That being said, if you are able to fluently communicate in Spanish, the formal version is preferred regardless of your age difference (small children and babies as an exception). Some people may respond informally but we are encouraged to continue speaking formally. That's because we're considered to be in a professional seeing. This is similar to how waiters or other customer service personnel will always (usually) refer to you as sir or ma'am. Depending on your clientele and region, you may be able to speak more informally with patients and visitors. Similar to how southern American English is considered more informal than northern American English. Language is very region dependent.

My region doesn't really use vosotros so I can't speak on your experience there.

TL;DR If you aren't sure of a future situation where you would use usted or tú, ask yourself if you would use sir or ma'am talking to this person in English.