Dave has it exactly right - there really is no set shelf-life for catamarans in general and hence, no specific age that should be avoided. Decide what your (realistically) expected use will be and, after doing some tire-kicking to assess the market, set a budget
for the boat/necessary upgrades.
seems to have the best supply of used cats, at least in North America. I would suggest that it would make a worthwhile trip, even if you are still not ready to actually take the plunge. You should be able to check out various models, of various ages, at various prices and get a better idea of the current
Thereafter you should be able to set a budget
and narrow down you options; at that point you can start seeking some boat-specific advice on this site and from various owner's associations/sites.
Good luck in your quest!