create table people
(   id int auto_increment primary key,
    name varchar(100) not null,
    birthdate timestamp not null -- uses a TIMESTAMP as requested
);
truncate table people;
insert people (name,birthdate) values ('John',current_timestamp());
insert people (name,birthdate) values ('Kim','1997-10-16');
insert people (name,birthdate) values ('James','1997-10-14');
insert people (name,birthdate) select 'Mary','1990-01-04';
insert people (name,birthdate) select 'Sally','1980-06-30';

select * from people;
+----+-------+---------------------+
| id | name  | birthdate           |
+----+-------+---------------------+
|  1 | John  | 2016-10-15 19:04:41 |
|  2 | Kim   | 1997-10-16 00:00:00 |
|  3 | James | 1997-10-14 00:00:00 |
|  4 | Mary  | 1990-01-04 00:00:00 |
|  5 | Sally | 1980-06-30 00:00:00 |
+----+-------+---------------------+

select id,
name,
birthdate,
datediff(now(),date_add(birthdate,interval 19 year)) as diffInDays
from people
having diffInDays>0;
+----+-------+---------------------+------------+
| id | name  | birthdate           | diffInDays |
+----+-------+---------------------+------------+
|  3 | James | 1997-10-14 00:00:00 |          1 |
|  4 | Mary  | 1990-01-04 00:00:00 |       2841 |
|  5 | Sally | 1980-06-30 00:00:00 |       6317 |
+----+-------+---------------------+------------+