Give me information and why you chose what you said as your answer please .
"Months" of age is an approximate concept at best, so its going to depend on the context. SE.Law has a discussion of the relevant bit of English law for contracts that specify a term of months, which is that:
if the period expired on a date that did not exist in a particular month (eg 31st February), then the period would expire on the last day of that month.
So in the UK a child born on 31st October would be legally one month old on the 30th November.