Unfortunately, I don't think anyone can accurately estimate how old your clam is. There are a few reasons for this. Closed marine systems are relatively unstable when compared to wild marine environments. (Excluding contamination and pollution by humans and temp. fluctuations due to El Nino)
Fluctuating elements, especially Ca, will dramatically affect the growth rate of your clam. Likewise, higher than natural nitrate and phosphate levels will increase shell growth rates unnaturally. All of these fluctuations take place in closed systems.
We do not know how long your squamosa has lived in captivity away from the ocean.
I can however, speculate on its age using information we do know. Squamosas are an intermediate species with regard to tridacnid growth rates. They are not "slow growers" like croceas, and they are not the "beasts of growth" we call derasa and gigas.
So, assuming normal growth rate in a stable marine environment, I estimate your clam to be somewhere in the range of 10 years old or so. This is a very general conservative guesstimate, but certainly he could have existed much longer than this because a 12" squamosa is reaching the edge of its maximum size. Growth rates would be much slower at this age, if he is adding new shell at all.
I would not be surprised if he reached 14" or 15", but from here on out is really an unknown. Assuming nothing happens to your clam he should live another 20 years or so with no problem.