In your experience, can old cats learn new tricks?

I'm just curious what others could lend from their experience, as to the ability of older cats to adjust. I've repeatedly heard the idea that if you want to do certain things, better get your cat used to it while they're a kitten or your chances of succeeding are reduced. 

I have two cats that are adult cats. I've never raised them for one minute of their kitten lives. My cats are not related in any way. They joined my family months apart through separate means.

One was about 14 when he joined our household and one was about 2 years old. 

a) I brush both of their sets of teeth thoroughly every day, spray their teeth with dental spray every day, and have never sustained any serious injuries doing so. 

b) I've harness/leash trained both of them.

c) Both of them continuously wear collars and ID tags.

So, in my experience, it wasn't hard at all to get my cats used to certain activities. However, I realize that my cats may be on the mellow side. 

So I was curious what other people have experienced with acclimating adult cats to new experiences.

I can tell you that I have brought 3 adult cats into my home in the last two years. Non of them were accustomed to other cats, dogs or kids. It took a lot of patience and understanding but they have adjusted well.

IMO, I think sometimes people try to make the transitions to quickly and the older cats are just overwhelmed. If they are given time I think an older cat can definitely come around and learn new behavior.