Well, people who join at 10 or even 5, can progress really fast. The levelling process starts to get prolonged around level 16. At this point I try to assist with battle when I can to speed it up a bit.
Now with regards to inactiveness, the clan does have rules it adheres to. People in general are expected to follow a levelling process that is compatible with professional and personal real - life.
When people are inactive for prolonged time periods, we are giving them some time, because it may be due to RL concerns. If the time passes, and they are still afk, we do ask them to leave the clan. The reason for that is that their place could be taken by someone who will be present in game and interacting with the clan members, making their game more enjoyable.
So in my opinion there is no reason to put level restrictions. However if more clan members express the same view, we could re-consider this.