Babies develop differently. Yours may be on the slow side, but don't worry about it. You should probably be able to download an eruption chart from
www.ada.org and you will see that eruption of teeth is a range of ages. I once saw a 22 month old that had no erupted teeth. But, I could feel and see bulges in the gums for them. About a month later the first one erupted (lower incisor) and within a few months, the baby had a mouthful. I have seen 9 year old children with either all baby teeth or all permanent teeth (except for wisom teeth). Even within families, the children will have different eruption patterns. My own kids were like that!