Age Defined Trigger Filter

Hi @SpinDr,

Welcome to the Community!

So for the age, you can only trigger a flow from a List, Segment, Date Property, or Metric. At this time there isn't an option to segment based on age out of the box from Klaviyo. 
However, if you're collecting age information from your customers separately (i.e. via a form) and storing this info in Klaviyo, you'll be able to create segments using that information. 
With that being said, if you wish to use a "customer age" as a trigger, it would depend on how that data is being passed into Klaviyo.
You mentioned you wished to collect that information at checkout so it would probably be passed through the Placed Order metric, but we don't know for sure until a someone has Placed an Order, input their customer age, then we can take a look at the raw data and see if that information is being passed through. If it is, you can use a trigger filter to distinguish the customer's age. Likewise if the data is being passed through by Shopify Tag you can use a Flow filter to find that information.

Additionally, before the purchase point, is it possible for you to collect their birthday information and go off that? If have not already collected birthday properties from your customers, then you can gather Birthdays via Signup Form by following this guide to Create a Signup Form: Overview of the Signup Form Builder​. Alternatively, if you want to collect birthdays via an email message, you can redirect recipients to the Manage Preferences/Subscribe Page within the email message, which is where they can update their birthday. If you are sending emails via FLOW, then the Manage Preferences form is located under the Preferences Pages tab in Klaviyo. Likewise, you could also redirect recipients to a List Subscribe Page, where they input their birthday and then are submitted to a specific List. Here are the instructions you'll need to follow: Collect Birthday via Subscribe/Preference Page.

Once you have collected at least one Birthday property that is recognized as a date by Klaviyo, you can follow this guide to segment customers by age: How to Create Segments Based on Birthdays. From that segment you could make different forms.

Hope this helped!
Alex