Mine is 18" and has been alive for 11-12 years.. I inherited it about 2 years ago.
I believe they are slow growers like due to stress and lack of proper nutrition.. They are hard to feed well, and they stress out from other fish in the tank.
If you kept one alone in a 55g, and fed it 2-3 cubes of bloodworms every day, it would grow alot faster.
Keep it in a community tank with competition and stress, it will grow alot slower..