Is it ever too late to introduce a new kitty to a resident cat?

Hello everyone.

I have a 10 year old neutered female, I've wanted to get a second cat for a long time but I don't know if it's too late now, I don't know if her age could make it too hard (or even impossible) to accept a new cat.
She's been my only cat, we had a Chihuahua dog when we got her as a kitten and even though they weren't friends they lived in peace for about 3 years, then the dog passed away.

We never attempted to get another pet until last year when a lovely female kitten of about 3 or 4 months of age showed at our door and we decided to keep her. I tried to follow a proper cat introduction process and after a couple days it seemed like my girl was starting to accept the new kitten but it was evident that she was still stressed and even when the kitten wasn't around she seemed to be super cautious, almost paranoid, like waiting for the kitten to appear at any moment. We didn't have a chance to see if she would get to accept the new kitty and go back to her relaxed self because the kitty ran away from us

She's always been rather territorial, she will confront any animal that approaches our front yard, though she doesn't get into fights but she makes it clear she doesn't want them near the house.

There's been a couple times in the past when we had to take care of 2 different friends' dogs (one being a little puppy) and my cat really seemed to hate them in both occasions, always trying to get close to them with no good intentions in mind. All these things make me wonder that, now that she's an older girl it would be really hard to get a new kitty. Am I wrong in my assumptions?

I'd like to get another cat but I wouldn't want to put my girl through a situation that may be too stressful for her. What do you think?

Thanks in advance for your answers.

Since she does exhibit territorial behavior it will be more difficult.  Yet it is not impossible.  She may not be happy at all about sharing her space, but if you do proper cat to cat introductions the chance for success is greater.  The process can take weeks to months to a year as it just depends on the tempermant of both cats.  If you resident cat is more sedentary and the kitten is high energy, the two may clash until they come to an understanding. 

You can try feliway diffusers to help as well as Composure calming treats or calming liquid.  You also need to be sure to have 3 litter boxes.  The golden rule is one per cat plus one extra.  You do not want to add litter box issues.

Here are some articles and a video on cat to cat intros so you can see if you would be able to put the effort into the process.  Most of us rush the process which leads to problems and leads to a longer period of adjustment.  I was one of those who rushed the process and had to start again.  My two took one solid year to adjust.  I had a very territorial cat which turned aggressive.  It took lots and lots of work.  After a year they could coexist and after 18 months they became buddies.  One cat was 5 and one was 3. 

http://www.thecatsite.com/a/the-ultimate-yet-simplified-guide-to-introducing-cats

http://www.catbehaviorassociates.com/a-simple-little-trick-to-use-during-new-cat-introductions/

http://www.aspca.org/pet-care/virtual-pet-behaviorist/cat-behavior/introducing-your-cat-new-cat

http://jacksongalaxy.com/2010/10/01/cat-to-cat-introductions/

In all honesty, I'm not sure I would put your girl through the stress of bringing in a new cat.

If you were to try it, you are more than likely looking at months to a year before she is willing to share her space.

You would also need to keep her and the new cat indoors only during the integration, otherwise one is likely to do just as the other kitten did and run away or find another home on their own.

If the integration isn't possible, would you be able and willing to allow the cats to just live separately?

Thank you both for your answers. @ShadowsRescue  thanks for the useful links!

I've got some research and lots of thinking to do, but I feel like the best option would be to not try introducing a new cat to our home.

I guess the wish of everyone getting a new cat is that the resident cat and the new one can becomes friends, I'd definitely like that, if that wasn't possible I think it would be ok if they could live in peace with each other, but since our house is small I don't know if it would be possible for the two cats too live "separate" lives.

Again, thank you both for your answers!