I'm no expert, but it sounds like you're doing everything right. As long as you keep it fun, I don't see age as a barrier.
Anyone questioning Yorkie intelligence should check out these vids from then-7-month-old Joey. His mama believes in clicker training and has done phenomenal things with positive reinforcement.
As for my experience, my Yorkie is the smartest dog I've ever owned.
http://www.dropshots.com/day.php?use...cdate=20060121
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Mike ~ Doting Dad to Jillie, Harper, Molly, Cooper, Eddie (RIP), Lucy (RIP), Rusty (RIP) and Jack (RIP).
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