As each family registration must consist of at least one adult, but not more than 2 we need a way to restrict the dates of birth for each member of the group registration.

For this we decided therefore that Registration 1 was to be the primary adult, Registraiton 2 could be the second adult or a child and Registrations 3 and 4 would be children. The layout explains how this was presented in the form to make it clear. 

In order to then filter the corresponding ages we setup multiple DateOfBirth fields which would show according to specific criteria, each with their own filter. 

The first was setup for the Single Registration which had no filters applied. 
The rest were setup to only show for Group Registrations and had additional filters applied.

The second was setup to Show only if Line = 1 this setting refers to the Group Registration position, and so this is field is only shown for the first group registration. We said that the first registration was to be the primary adult and so the validation rule was set to filter by age using:

[Age]>=18

The next DateOfBirth field was set to Show only if Line = 2, as this could be either adult or child, there was no validation rule set. 

The final DateOfBirth was set to Show only if Line >= 2, and so shows for registrations 3 and 4, and this also uses a data validation rule, since these registrations must be children

[Age]<18

With each of these setup now then a unique data validation rule can apply for individual registrations.