This is the 13155th most frequent Russian word.
месячным
"Месячным" means "monthly" or "related to a month" in English.
Here, "месячном" is a form of the word "месячный" used to describe something related to a month, modifying the word "возрасте" to indicate the age of the baby.
Ребёнок впервые улыбнулся в своём месячном возрасте.
The baby smiled for the first time at one month old.
In this sentence, "месячным" describes the frequency or period of the subscription, meaning that it renews every month.
Моя подписка на журнал является месячной.
My subscription to the magazine is monthly.
Here, "месячной" refers to the salary being calculated or received on a monthly basis, modifying the word "зарплате".