Harder to tame older hamsters?

He shouldn't be any harder to tame, it'll just take longer. Your poor hamster shouldn't have even had a cage mate in the pet shop at all so you were right to state your case, and I'd recommend you don't shop there anymore. Syrian hamsters are solitary creatures and be killed for entering another's territory. You've probably done this to tame your other hamsters but I'll run through the drill anyway, remember it'll take a while, Syrians take longer to tame anyway, so an abused Syrian will take a good few month maybe. Put your (clean) hand in the cage and leave it there for 10 minutes a day, don't try to pick him up and keep as still as possible. After a few weeks he should have came over for a sniff, maybe nipped once or twice (don't react to that at all, or he'll see it as a way to get rid of you, you don't want that), and then just carried on with his business as normal. Once he's doing that, or even a little earlier if he's responding well enough, you can put a treat on your hand, at first he'll just take it and run off, but once he spends a little time on your hand, or even sits there to eat you can try gradually lifting him. Get a bit higher every day until you can place him on your lap, and offer treats the whole time. A hamster can learn at any age, how long it'll take depends on their past experience.