As on a voyage when the vessel has reached a port, if you go out to get water it is an amusement by the way to pick up a shellfish or some bulb, but your thoughts ought to be directed to the ship, and you ought to be constantly watching if the captain should call, and then you must throw away all those things, that you may not be bound and pitched into the ship like sheep.
So in life also, if there be given to you instead of a little bulb and a shell a wife and child, there will be nothing to prevent you from taking them. But if the captain should call, run to the ship and leave all those things without regard to them.
But if you are old, do not even go far from the ship, lest when you are called you make default
The Stoics beleived in fate and God, and I take this metaphor to be a metaphysical one, with the captain being either God or fate calling you. But the underlying concept doesn't rely on such a beleif. What I take from this is: enjoy life's pleasures when they are available, but be prepared for them not to last.
I'm still not quite sure what the last line means. Perhaps it means that, as you near the end of life, be more careful in your Stoicism because you have less time to achieve mastery. This strikes me as strange; Stoicism isn't focused on any sort of afterlife, but on living a good an joyful life. It's also a philosphy that focuses on being present: as Marcus Aureilius says: "live every day as if it is your last". In that sense I'm not sure what differece it makes how long you have to live.