I have a yorkie and was wondering about another dog sometime in the future, but i was wondering at what age is it really better for my resident dog to be before introducing a new friend?

It is best that they are a mature well trained adult.  That way the new pup will respect them, and they will more easily be able to handle a pup 

Also the training shoudl be as good as you would like it before adding a new one, as some slipping backwards will occur, and you want them to set a good example for the new pup not have two hoolignas :D

In my own medium sized breed I would say 3 years ideally but at least two.

I agree - 2-3 years is a good interval between pups. through circumstances beyond my control, I presently have a 10 year gap!

Well if you know you will only want two dogs ideally get the secodn about half way through the average lifespan then you always have one in theory when the older one dies.

Thanks for replying, our yourkie is only a babe still at the moment, 5 months old, and i have a one year old baby too, so we were thinking of getting another dog when my baby is about 3 yrs, which will make the yorkie nearly three.
I was just wondering if that would be a sufficiant age gap, and you have all confirmed it for me , so thank you!