There's lots of information about all those topics around the forum
You just need to look in the different threads!
I'm not sure about how to tell their age, maybe someone else can answer that
To tame, you can offer them treats, like sunflower seeds, eventually they will eat out of your hand, it's a good idea to put them in a bath tub with a towel laid down and toys etc. Try picking them up and holding them, it can take a while but if you keep up to it, you should be able to hold them for longer.
Hamsters need a base mix of food, a mixture that has seeds in and peanuts etc. im not sure what kind are available where you live, but if you look at the diet section of the forum, im sure you can find what's best for you.
Hamsters can have vegetables in small amounts, but you have to build them up, you can give them a small bit the size of a fingernail once a week until they are used to eating it, then increase how often you give them it. They can have lettuce, bell pepper (no hot peppers!) carrot etc. There is a list on safe and dangerous foods in the diet thread too
Theres loads of different bedding you can use, i think the main one to avoid is wooden shavings as they can have harmful oils and also down think it's good for hamsters to breath in the dust etc.
The best is paper types of bedding, i know people use carefresh and some make their own! You just need to search around the forum and see what you think is the best for you.
Finally, i think you can tell a hamster is stressed if they aren't eating or drinking, basically if they aren't acting normal, there's probably something wrong, im not an expert on this bit but maybe someone else can answer this, or you could make a separate thread to ask the question
I hope some of this helped!