They have just been pampered! LOL! They all started out as strays, so could have been older - Bijou may be actually 19 or 20. We just guessed he was about a year old when we got him.
They have all been indoor/outdoor cats, because they were so used to being outdoors. I only ever let them out for short periods in the mornings before they ate, though! Bijou knows the routine!
I had a Siamese too who lived till he was 17 or 18. He also had been a stray!