When you see figures that say that on **average** the first primary teeth should be errupting between the ages of 4 and 6 months, well just like anything that is an average, you'll see some under it, and some over it.
My 11.5 month old just got his 2nd tooth (his first primary tooth showed up at just past 10 months), sodon't woory about it. If anything,you can look at it as with late errupting primary teeth will gnerally translate into late errupting permanent teeth, and the later that permanent teeth errupt, the more time for jaw growth to occurr, and the more space for permanent teeth to errupt into, the better!