if ( age>=21 && credit>=10000 )
The relational operators are both >= because the renter may be exactly 21 and may have exactly $10,000 credit.
Using > where greater-or-equal is required is a common mistake.
The customer must get true for both the age test and the credit test. If both tests are passed, the && combines the two true's into true.
A 24 year old customer with $0 credit could not rent a car. The boolean expression looks like this:
age >= 21 && credit >= 10000
--------- ---------------
true false
----------------
false
A 19 year old customer with $500000 credit could not rent a car. The boolean expression looks like this:
age >= 21 && credit >= 10000
--------- ---------------
false true
----------------
false
Could a 30 year old with $10000 credit rent a car?