Welcome to the forum and to RES care!
It can take RES several days to get used to their new surroundings and start eating. I would say keep offering pellets for now. It's a bit soon to be offering tuna, I'd say. If you start giving him tuna now, he'll just expect it and it'll be even harder to get him onto pellets and veggies.
After a few days, if he's still not eating, there are some methods to entice eating here:
http://www.redearslider.com/entice_eating.html
Spinach is not a good food for RES. It's high in oxylic acid, and too much can block calcium absorption.
Check out the nutrition section on the main site (
http://www.redearslider.com/index_nutrition.html) and the stickies at the top of this forum for what to feed and what not to feed.
On age/size/gender: You can't tell a RES's age by size. They are usually 3.5-4" at 1 year old, and grow approximately .5" per year after that. Those are just guidelines though. Size is dependent on many factors including habitat, nutrition and genetics.
At 1.5", I'd hazard a guess and say your RES is 6months or younger. Also, at that size, it's impossible to tell gender. You usually can't tell a RES's gender until s/he is at least 4".