Perhaps I should clarify... It is best for all of the batteries to be the same age. Basically the chemistry going on inside the battery slowly changes over time (sulfation, etc.) However this typically means you should not mix batteries that a number of years old with a new battery. I would have a hard time believing that a few weeks difference is going to make any real difference, especially if you can keep the two you have on a modern battery charger
as much as reasonably possible. If you do need to disconnect from power (like to go sailing) be sure to recharge before reaching 50% of the amp hours rating. This assumes you have a battery monitoring system that tells you amp hours used. If not you should get one. This is far more important than a few weeks difference in battery age.