I have this, which seems to output what I want. Now I need to put the 2 cases into a function I guess

SELECT
    gender,
    CASE
      WHEN age IS NULL
        THEN NULL
      WHEN age >= 55
          THEN 55
      WHEN age >= 45
        THEN 45
      WHEN age >= 36
        THEN 36
      WHEN age >= 30
        THEN 30
      WHEN age >= 25
        THEN 25
      WHEN age >= 18
        THEN 18
      ELSE NULL
    END
      AS "age_from",
    CASE
      WHEN age IS NULL
        THEN NULL
      WHEN age <= 17
          THEN 17
      WHEN age <= 24
        THEN 24
      WHEN age <= 29
        THEN 29
      WHEN age <= 35
        THEN 35
      WHEN age <= 44
        THEN 44
      WHEN age <= 54
        THEN 54
      ELSE NULL
    END
      AS "age_to",
    COUNT(population.id) AS "count"
  FROM population
  join people on population.person_id = people.id
  where period_id = 13
  GROUP BY
    period_id,
    age_to,
    gender

  ORDER BY age_from ASC

I would create a view for that using if conditions.