They stopped recommending further boosters to younger people after the 1st booster. These 50+ or 65+ only recommendations are rather recent. That's all I'm saying. You would need to account only from the 2nd booster onwards basically.
Which country(s) specifically changed their guidance about boosters after the 1st booster was released? Again I'm not aware of large swings in policy in most of these countries. Some aren't recommending 4th doses yet but I don't think any have changed policies about boosters after the fact.