Thrifty's Panama web site says minimum age 21, no mention of surcharge. Budget says $3/day extra if you're under 25. Didn't check any others.
at Budgetpanama.com, their website says minimum age is 25 (no options for under 25) and the thrift website says there is a surcharge of 9.99/day. but at least that is an option albeit an expensive one.
For Budget, see this page:
You're right about Thrifty; sorry I missed that.
Avis says 21 is OK if you use Amex.
You can research others from this starting point:
But, Panama businesses are famous for lousy internet presence; you may have to call them to get definite answers.
An afterthought: Are your interviews in Panama City? If yes, you don't need (nor want) a rental car; taxis are the way to go. Rent a car only for exploring outside Panama City.
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