The IRCC clearly states that in such cases the "Lock in date" will be when the Province received my application.
IRCC Express Entry guidance: age lock-in dates. This is the link for guidelines about the Lock in Date
Hope this clarifies.
This proves that the lock in date was after your son was 22 once your application received a nomination and you submitted your application. Submitting your application to be in a pool to be possibly selected for PR does not create a lock-in date.
