I've just had a look on the net and apparently an American quarter is about 2.5cm in diameter.
That makes the scorpion pictured roughly 3.5cm in length. At that size I'd say it was probably in its 3rd instar, maybe 4th. I'm not sure how many moults it takes S. mesaensis to reach maturity but assuming it is 7, then it has 3 or 4 more moults left to adulthood. The scorpion should reach around 8cm as an adult.
As for how old it is, I have no idea. I don't know how long it usually takes for S. mesaensis to reach that size, and besides, scorpions grow at different rates depending on environmental conditions, food availability etc. So, unless you know the date it was born, it is impossible to give a definite age.