How to setup plan with age restrictions

Hello Experts,

I currently have a medical plan which will trigger the loss of eligibility LE once the child reaches 26. I do this through the reevaluate designee eligibility process.

We have a requirement to have a plan switch options automatically once a child reaches the age of 26. I wanted to see what is the best way to go about this? I did come across some threads suggesting using Default Enrollment Fast formula. Please share any ideas! Thank you all ?