When is a personal choice, of course.
A quick story on the topic. The boat pictured is our 48 foot Alden designed yawl, THETIS ex Hermita. She was built in 1921 by Pendleton in Wiscasset, Maine. Mom and dad bought the boat in the winter of 1958 and refitted her for a voyage to the Bahamas which we began in 1960. I was 10 and my siblings were 9, 7 and 3. Mom and dad were 34 years old. They had been saving for and planning the cruise since before marriage.
So, what's the right age to go cruising? I think it has more to do with when you are ready than how old you are. As a now 70 year old, the upper limits are talking to me.
While cruising, we met a lot of cruising families with kids our ages and parents who were contemporaries of my parents. I remember the yacht names, Black Dragon, Black Pearl, Caravan, and Falcon plus a few others that slip my mind. I remember the times as having more and better friends while cruising than while living on Cape Cod.
Ready. Set. Go.