It should increase in salinity level as it matures. Some people will say otherwise, and some have even been kept to full size in FW. Your puffer will be alot healthier and happier if you give it the proper salinity level, and it will live quite a long time. Heres a thread on GSP.
miles is right agian!!! many of the ones i sell are full adults in FW but I feel even as a FW guy the adults will do a lot better in full out salty nastiness (sorry miles don't like the marine tanks)