Why does innodb next-key locks lock more range than the condition?

DROP TABLE IF EXISTS person;
CREATE TABLE person (
    id   int unsigned auto_increment primary key,
    name varchar(255) not null,
    age  tinyint unsigned,
    key (age)
);

INSERT INTO person (name, age)
VALUES ('bob', 10), ('alice', 16), ('jack', 19), ('william', 20);

begin;
select * from person where age < 12 for update;
rollback;

select LOCK_TYPE, LOCK_MODE, LOCK_STATUS, LOCK_DATA 
from performance_schema.data_locks;

TABLE,IX,GRANTED,
RECORD,X,GRANTED,"10, 1"
RECORD,X,GRANTED,"16, 2"
RECORD,"X,REC_NOT_GAP",GRANTED,1