A couple of things:
1. The FLOOR function will return the same data type as the expression being floored. What that data type is in your case, I'm not sure. It might even be a float but, I suspect it's Numeric. If it is a Float/Real, that might cause comparison issues.
2. You have wrapped the value-literal in single quotes ('21') that denotes a string. I'd have to look up the casting precedence to know for sure what SQL is doing, but that is probably your issue. You should not rely on implicit casting by SQL, rather you should explicitly cast the to the data type you want it to be.
CAST(((sysdate-birthdate)/365.25) AS INT) >= 21