Is it legal to fly a commercial aircraft at 20 years old?

Yes, flying a commercial aircraft at 20 is legal under FAA rules, though it comes with a ceiling: the co-pilot role on a charter flight like the one Nathalia Gadelha Filgueiras was aboard sits within what a commercial certificate permits, but the highest-level certificate required to fly for the major airlines carries a higher age floor.

What each certificate allows

The FAA (Federal Aviation Administration, the agency that sets the rules for all civil aviation in the US) runs a three-rung ladder:

• A private pilot certificate requires a minimum age of 17, and a commercial pilot certificate requires a minimum age of 18, under FAA rules 14 CFR §61.103 and §61.123 respectively.
• The Airline Transport Pilot (ATP) certificate, the highest level of FAA pilot certification and what major airlines require before putting anyone in a cockpit on scheduled passenger service, carries a standard minimum age of 23 under 14 CFR §61.153.

A commercial certificate is what you need to be paid to fly. The ATP is what comes next, when you're ready for the airlines.

Where Nathalia Gadelha fit in that ladder

She began flight lessons in 2020 under her father's guidance, earned her private pilot's license at 17 while attending West Boca Raton High School, and later graduated from American Flyers flight school.

The ATP certificate and the airlines themselves would have been her next step, available to her at 23.

The question investigators will ask

Whether Gadelha held the specific type rating for the Learjet 60, a separate qualification beyond the base certificate that certifies a pilot on a particular aircraft model, is what the NTSB (National Transportation Safety Board, the federal agency that investigates aviation accidents) will examine. Age alone was not a legal barrier to her role. What will matter is whether every certificate, rating and flight-hour requirement for that specific seat on that specific aircraft was met, and the NTSB investigation is ongoing.