Hi,

1. Newid error occurs when the compute field is not stored and When creating a record, the records of the recordset will be in memory only. At that time, the id of the record will be a dummy ids of type NewId

2. You need to assign empty Value before the Loop
    All the computed fields need to be assigned an empty value at least and its better if you use @api.mulit()

@api.mulit()
@api.depends('date_release')
def _compute_age(self):
    age_days = 0
    for book in self.filtered(lambda x: x.date_release):
        today = fields.Date.today()
        delta = today - book.date_release
        age_days = delta.days
        book.age_days = age_days

Thanks