MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table test (j json)
insert into test values 
('{
  "about": "person",
  "info": [
    {
      "fName": "John",
      "lName": "Booker",
      "sex": "male",
      "age": 20
    },
    {
      "fName": "Laurie",
      "lName": "Sparks",
      "sex": "female"
    },
    {
      "fName": "Adam",
      "lName": "Bate",
      "age": 26
    }
  ]
}')
SELECT JSON_ARRAYAGG(JSON_OBJECT('sex', j.sex, 'age', j.age)) AS filtered
FROM test t
JOIN JSON_TABLE(t.j, 
                '$.info[*]'
                COLUMNS (
                  sex VARCHAR(6) PATH '$.sex',
                  age INT PATH '$.age'
                )
                ) j


filtered


[{"age": 20, "sex": "male"}, {"age": null, "sex": "female"}, {"age": 26, "sex": null}]


SELECT JSON_ARRAYAGG(
         CASE WHEN j.sex IS NULL THEN JSON_OBJECT('age', j.age)
              WHEN j.age IS NULL THEN JSON_OBJECT('sex', j.sex)
              ELSE JSON_OBJECT('sex', j.sex, 'age', j.age)
         END
         ) AS filtered
FROM test t
JOIN JSON_TABLE(t.j, 
                '$.info[*]'
                COLUMNS (
                  sex VARCHAR(6) PATH '$.sex',
                  age INT PATH '$.age'
                )
                ) j
WHERE j.sex IS NOT NULL OR j.age IS NOT NULL


filtered


[{"age": 20, "sex": "male"}, {"sex": "female"}, {"age": 26}]