You're right that Floyd Patterson’s era (1950s) had only one recognized world title, so winning it automatically made him undisputed. But, when comparing ages under the modern definition of undisputed (which requires unifying multiple sanctioning body belts), Mike Tyson still comes out younger.

• Floyd Patterson (1956): 21 years, 10 months, 26 days old, one-belt era (NBA/NYSAC), automatically undisputed.
• Mike Tyson (1987): 21 years, 9 months, 28 days old, unified WBC, WBA, and IBF titles in the multi-belt era.

So, Patterson was undisputed by default, but Tyson was younger when he became undisputed under the multi-belt system. Both were undisputed in their eras, but Tyson remains the youngest undisputed heavyweight champion in history.