@Oleski ,
Which step of the process are you stuck on? I would make sure you first have a birthday property that appears under a user’s profile.
You will need to first create a custom property on profiles for birthdate using a flow. Then, you can use a signup form to get the birthdate. Once you have at least one birthdate populated. You can build your birthday flow using the custom field for birthdate.
Regarding a birthday flow, the birthday flow will not focus on the year of the customer's birthday, but rather the month and day. A birthday flow can be set to repeat yearly and can be sent on or before a person's birthday, which is the way your current birthday flow is set below:

For that reason, there would not be a way to insert a customer’s age for each flow using default signup forms and instead would have to be something utilizing a 3rd party app. Another way that this could be done in Klaviyo is to create a separate Age property with a numerical value and filter on that. Since birthday flows can repeat yearly they only really take into account the month and date, so there wouldn't be a way within a standard flow to filter on age. The only other way we could see this working is a script that pulls all the profiles birthdays, calculates the age and then updates the age profile property (which wouldn't be too difficult!). Especially now that we acquired https://www.napkin.io/
Hope that helps,
Alex