I also support the earlier comment that an age restriction should be able to be 'as of' a particular day of the year (at least).
This would certainly help our tennis club. I particularly support the earlier comment that the admin should be made aware of members progressing out of their current level (or becoming eligible for a new/discounted level) and requiring a change to a new level. This is the real benefit. The admin may validate the level at time of joining, but not constantly thereafter.
YES! - I would expect many sporting clubs would have at least some age restricted membership levels (e.g. Juniors). Our tennis club certainly does. While it is good to be able to see a member's age, the membership level should be able to have an age restriction defined. Also, age restrictions are often defined 'as of a specific day of the year'.
Our tennis club has family memberships where age restrictions apply to the 'children'. There may of course be two parents in the bundle. It would also be useful for the admin(s) to be made aware when age restrictions are crossed by existing members.