Is there a legal maximum altitude?
There's no legal limit of how high you're allowed to fly by law: there's no law that says


  It is illegal to fly above X,000ft


In some juristictions there are more specific laws, eg


  It is illegal to operate an unpressurised aircraft above 25,000ft


However, each aircraft has a service ceiling when it is certified, and it would be generally illegal to operate the aircraft above this altitude (because it's unsafe).

I'm fairly confident that you weren't at 55,000ft. No currently-operating commercial airliner I know of has a service ceiling higher than 45,000ft (the newer -800 version of the 747) and they almost never operate above 41,000ft. A few private jets can go a little higher, up to around 50,000ft, but I know of no non-military/experimental aircraft which is currently capable of going much above 50,000ft, even if you were daft enough to try.