FA 2023 (pg 684) says it’s age dependent “A-a gradient = Pao2 – Pao2. Normal A-a gradient estimated as (age/4) + 4 (eg, for a person < 40 years old, gradient should be < 14).” Amboss also gives this age-based calculation but also says: “5–10 mm Hg for a young person breathing room air at sea level
15–20 mm Hg in healthy older adult”