Firstly I'd like to wish you good luck with adding to your feline family.
Personally, I believe that the key to success is in the personalities of the animals themselves, not all cats are going to get along, as indeed not all people do.
Age can help to make a transition smoother, in the respect that a younger kitty can usually get away with more initially. But even then, it's not a guarantee and if your baby's used to being the only cat in the house then any addition could prove stressful at first, until they've had time to get used to each other, gradually and at their own pace.
I'll give you an example.
We have 3 bengals. We adopted Sasha (mum) first and she settled down quickly. WE then adopted one of her babies a couple of months later (Storm) and things went smoothly, they instantly recognised each other and took immediately. At the end of that year which was several months after getting Sasha and Storm we agreed to adopt the last of her litter (Cookie). Sasha was hissing at him at first but not aggressive. That in itself was enough to show us that she knew who he was and after a couple weeks of slow introductions she was fine with him. The problem was more Storm. She was terrified of him! We kept Cookie in a separate room for the first few weeks to allow him to settle down in a calm environment and everytime we took Storm in for introductions she went mad! He was so desperate to go and play with her as he's friendly to everything he meets but Storm is a big ball of nerves and wasn't having any of it. It took several weeks for her to come round to him but eventually she did and now they're all settled and you'd think they'd never been separated the way they all get along so well.
Obviously as they were all part of the same family we had an advantage but the timing between the girls and cookie was several months and he was also pretty much a year old by the time we took him in, so it was pretty much like starting a fresh. Relationships had to be re-established and the best way to do that with cats of any age is gradually.
Good luck, hope it all goes well for you. xxx