Before you get all excited, we don’t have any beta codes. This is just a public service announcement. 
Right, now that we have that out of the way here is how you have to set up to get e-mails from Sony containing beta codes in the first place.
Here is handy step-by-step guide:
1. Log into the PSN network.
2. Under “Playstation Network”, click on “Account Management”.
3. Click on “Account Information”.
4. Scroll down and select “Notification Preferences”.
5. On the following screen, make sure that you have both boxes ticked/checked. (This page also reminds you what email address is associated with your PSN.)
6. Click on “Confirm”.
7. Update completed! So just click “Ok”.
Of course you need a working e-mail address as well and you may want to set your account information so that it says that you are at least 18. 
Update: As has been pointed out by several people you can’t change your age. 