At what age do they start to blister? Should I sell her immediately?

Some boats can develope blisters at the waterline within a few years or sooner.
There was a run of Valiant yachts that had this problem reported within a year
of production. If you have blisters they will be small, as in the Jeaneau
problems a few years back. Of course that was thanks to Beneteau taking over!

RB