My watch say ECG not intended for under 22

Thanks for asking about the new ECG feature.

Check here for availability in your region: ECG

ECG requires the current non-beta version of iOS and watchOS. For additional help with the ECG feature on your Apple Watch, take a look at these articles.

Heart rate notifications on your Apple Watch - Apple Support

Taking an ECG with the ECG app on Apple Watch Series 4 or later - Apple Support

If you have any other questions please reach out to our team here: https://apple.co/HeartFeatures. If you believe that you are experiencing a medical emergency, please dial your local emergency services.

Take care 

Go into the watch app on the phone and tap on Health/Health Profile and tap on edit in the top right corner enter your details in here,birthdate,sex,height,weight and whether you're a wheelchair user then tap on done in the top right corner,and then restart the phone.

You need to enter you age,gender,height and weight in the watch app on the phone.