Age calulate age using check Constraint in Mysql

Now I want to calculate Age in Student table .I want to insert student record only greater then 18 year old .

mysql >alter table student add column  Age double (18,2) check (Age >=18);

Action Output:  0 row(s) affected Records: 0  Duplicates: 0  Warnings: 0    0.066 sec

mysql > select * from student;

Action Output: 

# ID,      DOB,         STUDENT_NAME, gender_id , Age
     '1', '2020-01-01',         'A',                      '1'        , NULL
 

Solution 1 :

Now update Age in Student table :

CREATE TABLE `student` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `DOB` date DEFAULT NULL,
  `STUDENT_NAME` varchar(10) DEFAULT NULL,
  `gender_id` int(11) DEFAULT NULL,
  `Age` int(11) DEFAULT NULL, `CHECK` ( AGE >=18 ),
  PRIMARY KEY (`ID`),
  UNIQUE KEY `IDDOB` (`ID`,`DOB`),
  KEY `fk_gender_id` (`gender_id`),
  CONSTRAINT `fk_gender_id` FOREIGN KEY (`gender_id`) REFERENCES `gender` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
 

Year Count :

mysql >set @dob='2000-01-01' 

             select year(now())-year(@dob);

Output: 20

mysql >set @dob='2000-01-01' 

 select date_format(from_days(datediff(now(),@dob)),'%Y')+ 0 ;

Output# date_format(from_days(datediff(now(),@dob)),'%Y')+ 0
'20'