To be getting long in the tooth means getting old, especially when talking about a person or an animal. It can also be used to talk about machines or systems that are old and may be past their best years. The expression comes from the way a horse's teeth change as it ages. As the gums slowly recede, more of the teeth become visible, making them look longer and showing that the horse is older.
That old farm truck is getting a bit long in the tooth.