In the era they were born the registration of births was often delayed and the hand-written Birth Certificates on soft paper liable to self-destruct over time. The recorded details from information supplied by parents, usually illiterate, resulted, in many cases, with no date of birth and or no month of birth being recorded only the year.
For example, my own wife, now 76 has only the Month and Year recorded. Her younger sister has only the Year recorded. Their Thai passports reflect this.
The true age of these centenials will never be known and are often exaggerated. My wife's late aunt made it to 107 but by then she was blind yet mobile.
Good luck to them.