It's next to impossible to determine age based solely on size and growth/growth rate. The only way you can ever know "for sure" is if you have proof from the breeder. A tiny sav can be stunted from very poor husbandry or it could just be young.
Most likely your Sav. is not captive bred. Most (seriously, almost all minus a few exceptions if you're very lucky and willing to pay a lot more...) of the commercially available savs are wild caught.
Even with good pictures, you may not know how old your monitor "really" is. Although, it sounds like he (or she) has grown substantially since you have had it...so I suppose you can assume it IS young. Keeping in mind, it's just an assumption, a likely assumption, but still not a definite.
sorry for not being more specific. That's just how it is sometimes 