States regulate this in the United States, but most states more or less follow the recommendations of the National Fire Protection Assocation (publication 58) for common DOT cylinders. After date of initial manufacture (month/year stamped in collar of cylinder), it's 12 years. After a visual recertification, it's five years (month/year with "E" stamped or labeled).
Permanently mounted cylinders as on motorhomes follow different rules as they are ASME cylinders, not DOT. There is no recertification requirement. However, fillers in most states are warned not to fill any cylinder (DOT or ASME) that looks damaged in any way.
:)
Lynn
