Haven't tried this trick yet. However, it should work for you with just the search box. Why? Because while the join page does take a date of birth, it converts it to an age to determine if the end user is of the age the webmaster has set for joining. So that check is already in place.
Moving on, you can make a simple adjustment to the code, do a search for:
date of birth v age
You will find a modification that was made to the code last fall/winter that displays the age instead on the profiles. I know some sell the modifcation, but you don't need to buy it as it's free since it was developed by the Unity Community for everyone. I've tested and it works perfectly on 6.1.6 still.
Just play with your settings and you should be able to get this to work without a lot of extra coding.
****Note: The above post was made by the New, Improved, Kinder, Gentler Mydatery and in no way is meant to insult, belittle, defamate, humiliate or intimidate any reader of this post. If you have further questions please feel free to ask and they will be answered in the same non-emotional, non-exciting, non-sarcastic, non-humorous manner and delivered just as boringly. Let us all Pray that the Gods will rain down on us a million users each and cause all of our servers to err due to over use.