Birth records can be found in multiple places such as county birth , hospital , church , census , marriage , immigration , school , military and death records. Tombstones, and journals are other sources.
If a record of birth date cannot be found an estimate the birth year can be done by using records and making a guess. An example might be that you find females parent’s names on a marriage record, but you have no dates for the parents. Most parents are generally 20 to 30 years older than their children. So subtract about 25 years from the females age and this will give you an estimated date of birth for the parents.